...for curl-config and its corresponding test 1014

Automake gets confused if you want to use C++ static libraries with C
code - basically we need to involve the clang++ linker. The easiest way
of achieving this is to rename the C code as C++ code. This gets us a
bit further along the path and ought to be compatible with Google's
version of clang.

- Start with the basic code from the ossfuzz project.
- Rewrite fuzz corpora to be binary files full of Type-Length-Value
  data, and write a glue layer in the fuzzing function to convert
  corpora into CURL options.
- Have supporting functions to generate corpora from existing tests
- Integrate with Makefile.am

... when darwinssl is used.

Reported-by: Viktor Szakats
Bug: https://github.com/curl/curl/commit/b0989cd3abaff4f9a0717b4875022fa79e33b481#commitcomment-23943493

Closes #1845

There is a mode in which libcurl is compiled with versioned symbols,
depending on the active SSL backend.

When multiple SSL backends are active, it does not make sense to favor
one over the others, so let's not: introduce a new prefix for the case
where multiple SSL backends are compiled into cURL.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Previously, we used as default SSL backend whatever was first in the
`available_backends` array.

However, some users may want to override that default without patching
the source code.

Now they can: with the --with-default-ssl-backend=<backend> option of
the ./configure script.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

This new feature flag reports When cURL was built with multiple SSL
backends.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

When building software for the masses, it is sometimes not possible to
decide for all users which SSL backend is appropriate.

Git for Windows, for example,  uses cURL to perform clones, fetches and
pushes via HTTPS, and some users strongly prefer OpenSSL, while other
users really need to use Secure Channel because it offers
enterprise-ready tools to manage credentials via Windows' Credential
Store.

The current Git for Windows versions use the ugly work-around of
building libcurl once with OpenSSL support and once with Secure Channel
support, and switching out the binaries in the installer depending on
the user's choice.

Needless to say, this is a super ugly workaround that actually only
works in some cases: Git for Windows also comes in a portable form, and
in a form intended for third-party applications requiring Git
functionality, in which cases this "swap out libcurl-4.dll" simply is
not an option.

Therefore, the Git for Windows project has a vested interest in teaching
cURL to make the SSL backend a *runtime* option.

This patch makes that possible.

By running ./configure with multiple --with-<backend> options, cURL will
be built with multiple backends.

For the moment, the backend can be configured using the environment
variable CURL_SSL_BACKEND (valid values are e.g. "openssl" and
"schannel").

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

... as they're not used externally and internally we check for the sizes
already in configure etc.

Closes #1767

This change does two things:
1. It un-breaks the build in Xcode 9.0. (Xcode 9.0 is currently
   failing trying to compile connectx() in lib/connect.c.)
2. It finally weak-links the connectx() function, and falls back on
   connect() when run on older operating systems.

Closes #1647

Fixes #1669
Closes #1713

fork, getprotobyname, inet_addr, perror, uname

closes #1638

Rely entirely on curl/system.h now.

Introduced in Aug 2008 with commit 14240e9e. Now gone.

Fixes #1456

Bug: https://curl.haxx.se/mail/lib-2017-06/0017.html

Closes #1528

- Change prepends to appends because user's LDFLAGS and CPPFLAGS should
  always come first so they're searched before ours.

Bug: https://github.com/curl/curl/issues/1420
Reported-by: Helmut K. C. Tessarek

The check for if -ldl is needed to build with (a statically built)
openssl was broken. This repairs the check, and adds a check for
-lpthread as well since OpenSSL 1.1.0+ does in fact require -lpthread so
only adding -ldl for a static openssl build is no longer enough.

Reported-by: Jay Satiro
Ref: #1426
Closes #1427

Ignore everything after the version numbers in LIBCURL_VERSION and
LIBCURL_VERSION_NUM to ged rid of the extra CR character.
This makes tests 1022 and 1023 pass on Linux with a CRLF checkout.

Ref: https://github.com/curl/curl/pull/1344#issuecomment-289243166
Closes https://github.com/curl/curl/pull/1422

Prior to this change if you attempted to configure curl using
--wtih-zlib and specified a path the path would be ignored if you also
had pkg-config installed on your system.  This situation can easily
arise when you are cross compiling.  This change moves the test for
detecting zlib settings via pkg-config only if OPT_ZLIB is not set.

Closes https://github.com/curl/curl/pull/1292

Better handle options conflicts that can occur if --enable-pthreads.

Bug: https://github.com/curl/curl/pull/1295
Reported-by: Marc-Antoine Perennou

When the threaded resolver option is specified for configure the default
thread library is pthreads. This change makes it possible to
--disable-pthreads and then configure can fall back on Win32 threads for
native Windows builds.

Closes https://github.com/curl/curl/pull/1260

Check for presence of gnutls_alpn_* and gnutls_ocsp_* functions during
configure instead of relying on the version number.  GnuTLS has options
to turn these features off and we ca just work with with such builds
like we work with older versions.

Signed-off-by: Marcus Hoffmann <m.hoffmann@cartelsol.com>

Closes #1204

... which the help text already implied since we switched to libidn2
from libidn in commit 9c91ec77 back in October 2016.

Reported-by: Christian Weisgerber
Bug: https://curl.haxx.se/mail/lib-2016-12/0110.html

Closes #1142

and remove RAND_screen from configure since nothing is using that
function

CVE-2016-8625

Bug: https://curl.haxx.se/docs/adv_20161102K.html
Reported-by: Christian Heimes

This helps building binaries that can work on multiple macOS versions.

Help-by: Martin Storsjö

Fixes #1069

The tool was never called cURL, only the project. But even so, we have
more and more over time switched to just use lower case.

A libssh2 library in the standard system location was being used in
preference to the desired one while linking.

The OpenSSL function CRYTPO_cleanup_all_ex_data() cannot be called
multiple times without crashing - and other libs might call it! We
basically cannot call it without risking a crash. The function is a
no-op since OpenSSL 1.1.0.

Not calling this function only risks a small memory leak with OpenSSL <
1.1.0.

Bug: https://curl.haxx.se/mail/lib-2016-09/0045.html
Reported-by: Todd Short

Since I first wrote that text, Apple introduced tvOS and watchOS, and renamed "Mac OS X" to "macOS." Let's make the text a little more inclusive, since curl can be built for all four operating systems.

... instead of relying on the pkg-config autoconf macros to be present.

Fixes #972 (again...)

With commit c2f9b78a we added a new dependency on pkg-config for
developers which may be unwanted. This change make the configure script
still work as before if pkg-config isn't installed, it'll just use the
old zlib detection logic without pkg-config.

Reported-by: Marc Hörsken

Fixes #972