add default USE_CURLDEBUG setting

initial step towards decoupling c-ares from libcurl for DOS

improve usability with UNIX-like shells or a DOS command interpreters

don't ignore these subdirs, they must be removed first

fix comment

Try to make more clear that --enable-curldebug has nothing to do with --enable-debug for this library.

Revert last change, it is inappropriate.

fix compiler warning

Fixed to take in account the different interpretation of double
quootes on UNIX-like shells vs DOS-like command interpreters.

include <limits.h> for INT_MAX definition

fix compiler warning

fix compiler warning

when running automake copy missing files instead of symlinking them

DOS adjustments

added header inclusion guard

mention configure --enable-curldebug decoupled from --enable-debug

changed testcurl script to allow building test harness
programs when cross-compiling for a *-*-mingw* host.

  contributed a range of patches to fix them.

With the curl memory tracking feature decoupled from the debug build feature,
CURLDEBUG and DEBUGBUILD preprocessor symbol definitions are used as follows:

CURLDEBUG used for curl debug memory tracking specific code (--enable-curldebug)

DEBUGBUILD used for debug enabled specific code (--enable-debug)

with the low-level curl debug memory tracking 'feature' to allow decoupled
setting from --enable-debug.

  issue with client certs that caused issues like segfaults.
  http://curl.haxx.se/mail/lib-2009-05/0316.html

  to detect gnutls build options with pkg-config only and not libgnutls-config
  anymore since GnuTLS has stopped distributing that tool. If an explicit path
  is given to configure, we will instead guess on how to link and use that
  lib. I did not use the patch from the bug report.