m4 quadrigraph shell comment technique allows proper autoconf
parentheses balancing in shell 'case' statements. The presence
of unbalanced parentheses may otherwise trigger expansion bugs.

- Update comments
- Execute commands in subshells
- Faster path separator check
- Fix missing 'test' command
- Rename private macros
- Minimize AC_REQUIRE usage

Some basic checks we make were placed early enough in generated
configure script when using autoconf 2.5X versions. Newer autoconf
versions expand these checks much further into the configure script,
rendering them useless. Using XC_CONFIGURE_PREAMBLE fixes placement
of early intended checks across all our autoconf supported versions.

Notes:

When running a configure script that has nested packages (for example
libcurl's configure with --enable-ares and c-ares sources embedded in
curl tree) and AC_CONFIG_SUBDIRS([nested-subdir]) machinery is used to
automatically run the nested configure script from within the parent
configure script, it happens that the nested _shell_ script will
inherit shell variables exported from the parent _shell_ script.

If for example parent configure script sets and exports LDFLAGS and LIBS
variables with proper values in order to link either a parent library or
program with a library which will be configured and built by a nested
package; It will happen that when the nested configure script runs, the
nested library does not exist yet and _any_ link-test done in the nested
configure will fail, such as those that autoconf macros perform in order
to detect existing compiler and its characteristics, the result is that
the nested configure script will fail with errors such as:

configure: error: C compiler cannot create executables

For now, we no longer export variables previously exported here.

On the other hand, AC_SUBST'ing them is appropriate and even with nested
packages each package's config.status gets its own package values.

So we reinstate AC_SUBST'ing previously AC_SUBST'ed variables.

Fix proper macro expansion order across autotools versions for
C compiler and preprocessor program checks.

Tested with:

buildconf: autoconf version 2.69
buildconf: autom4te version 2.69
buildconf: autoheader version 2.69
buildconf: automake version 1.13.1
buildconf: aclocal version 1.13.1
buildconf: libtool version 2.4
buildconf: GNU m4 version 1.4.16

This commit renames lib/setup.h to lib/curl_setup.h and
renames lib/setup_once.h to lib/curl_setup_once.h.

Removes the need and usage of a header inclusion guard foreign
to libcurl. [1]

Removes the need and presence of an alarming notice we carried
in old setup_once.h [2]

----------------------------------------

1 - lib/setup_once.h used __SETUP_ONCE_H macro as header inclusion guard
    up to commit ec691ca3 which changed this to HEADER_CURL_SETUP_ONCE_H,
    this single inclusion guard is enough to ensure that inclusion of
    lib/setup_once.h done from lib/setup.h is only done once.

    Additionally lib/setup.h has always used __SETUP_ONCE_H macro to
    protect inclusion of setup_once.h even after commit ec691ca3, this
    was to avoid a circular header inclusion triggered when building a
    c-ares enabled version with c-ares sources available which also has
    a setup_once.h header. Commit ec691ca3 exposes the real nature of
    __SETUP_ONCE_H usage in lib/setup.h, it is a header inclusion guard
    foreign to libcurl belonging to c-ares's setup_once.h

    The renaming this commit does, fixes the circular header inclusion,
    and as such removes the need and usage of a header inclusion guard
    foreign to libcurl. Macro __SETUP_ONCE_H no longer used in libcurl.

2 - Due to the circular interdependency of old lib/setup_once.h and the
    c-ares setup_once.h header, old file lib/setup_once.h has carried
    back from 2006 up to now days an alarming and prominent notice about
    the need of keeping libcurl's and c-ares's setup_once.h in sync.

    Given that this commit fixes the circular interdependency, the need
    and presence of mentioned notice is removed.

    All mentioned interdependencies come back from now old days when
    the c-ares project lived inside a curl subdirectory. This commit
    removes last traces of such fact.

This reverts renaming and usage of lib/*.h header files done
28-12-2012, reverting 2 commits:

  f871de00... build: make use of 76 lib/*.h renamed files
  ffd8e127... build: rename 76 lib/*.h files

This also reverts removal of redundant include guard (redundant thanks
to changes in above commits) done 2-12-2013, reverting 1 commit:

  c087374c... curl_setup.h: remove redundant include guard

This also reverts renaming and usage of lib/*.c source files done
3-12-2013, reverting 3 commits:

  13606bbf... build: make use of 93 lib/*.c renamed files
  5b6e7927... build: rename 93 lib/*.c files
  7d83dfff... build: commit 13606bbf follow-up 1

Start of related discussion thread:

  http://curl.haxx.se/mail/lib-2013-01/0012.html

Asking for confirmation on pushing this revertion commit:

  http://curl.haxx.se/mail/lib-2013-01/0048.html

Confirmation summary:

  http://curl.haxx.se/mail/lib-2013-01/0079.html

NOTICE: The list of 2 files that have been modified by other
intermixed commits, while renamed, and also by at least one
of the 6 commits this one reverts follows below. These 2 files
will exhibit a hole in history unless git's '--follow' option
is used when viewing logs.

  lib/curl_imap.h
  lib/curl_smtp.h

Provide a 'traceable' AC_CONFIG_MACRO_DIR definition only when using
an autoconf version that does not provide it, instead of what we were
doing up to now of providing and overriding AC_CONFIG_MACRO_DIR for
all autoconf versions.

76 private header files renamed to use our standard naming scheme.

This change affects 322 files in libcurl's source tree.

When nothing is told to configure, we should not enforce switching off
debug options with -g0 (or similar). We instead don't use -g at all in
that situaion and therefore allow the user's CFLAGS settings possibly
dictate what to do.

Removing this option as it currently only functions to lure people into
wrongly using it and falsely believing that libcurl will work fine
without using nonblocking sockets internally - which leads to hard to
track or understand errors.

Currently it is unknown if there is any version of clang that
actually supports -Wstrict-aliasing. What is known is that there
are several that don't support it.

When building a Windows target with gcc 4.5 or newer and strict compiler
warnings enabled use -Wno-pedantic-ms-format in addition to other flags.

This protects from attribute names being defined by third party's code.

Improvement: http://curl.haxx.se/mail/lib-2012-04/0127.html

configure script now provides conditional definitions for Makefile.am
that result in CURL_HIDDEN_SYMBOLS being defined by resulting makefiles
when appropriate.

Additionally, configure script option for symbol hiding control is now
named --enable-symbol-hiding --disable-symbol-hiding. While still valid,
old option name --enable-hidden-symbols --disable-hidden-symbols will
be deprecated in some future release.

Take in account that h_errno might be a modifiable lvalue not defined as
a C preprocessor macro

Take in account that POSIX standard Issue 7 drops h_errno support. Now, we also
consider getaddrinfo() to be thread-safe when (_POSIX_C_SOURCE >= 200809L) or
(_XOPEN_SOURCE >= 700) independently of whether h_errno exists or not.

It seems some versions of the OpenSSL version defines don't come with L
appended to the number, so let's deal with that nicely.

Configure script option --enable-wb-ntlm-auth renamed to --enable-ntlm-wb
Configure script option --disable-wb-ntlm-auth renamed to --disable-ntlm-wb

Preprocessor symbol WINBIND_NTLM_AUTH_ENABLED renamed to NTLM_WB_ENABLED
Preprocessor symbol WINBIND_NTLM_AUTH_FILE renamed to NTLM_WB_FILE

Test harness env var CURL_NTLM_AUTH renamed to CURL_NTLM_WB_FILE

Static function wb_ntlm_close renamed to ntlm_wb_cleanup
Static function wb_ntlm_initiate renamed to ntlm_wb_init
Static function wb_ntlm_response renamed to ntlm_wb_response

Feature string literal NTLM_SSO renamed to NTLM_WB.
Preprocessor symbol USE_NTLM_SSO renamed to WINBIND_NTLM_AUTH_ENABLED.
curl's 'long' option 'ntlm-sso' renamed to 'ntlm-wb'.
Fix some comments to make clear that this is actually a NTLM delegation.

Add CURL_CHECK_COMPILER as a requirement.

Ensure macro does nothing unless GNU_C or CLANG compiler is used.

This should allow usage of this macro in unforeseen placements.

Use preprocessor symbols WINBIND_NTLM_AUTH_ENABLED and WINBIND_NTLM_AUTH_FILE
for Samba's winbind daemon ntlm_auth helper code implementation and filename.

Retain preprocessor symbol USE_NTLM_SSO for NTLM single-sign-on feature
availability implementation independent.

For test harness, prefix NTLM_AUTH environment vars with CURL_

Refactor and rename configure option --with-ntlm-auth to --enable-wb-ntlm-auth[=FILE]

This reverts commit bc6e6a46.