Add some comments to better understand what the regex's pretend to achieve.

is only expanded and included once in the configure script.

due to autoconf 2.64 m4_require'ing it in _AS_SHELL_SANITIZE indirectly through
_AS_PATH_WALK.

Check for basename() is now done the same as other function checks

make errno available as a preprocessor macro.

renamed generated config.h to curl_config.h in order to avoid clashes when libcurl is used with other projects which also have a config.h.

Refactor how libraries are checked for connect() function,
and check for connect() as it is done for other functions.

socket() and closesocket() prototypes might be in socket.h

check for socket() and closesocket() as it is done for other functions

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

Moved *_CHECK_COMPILER_HALT_ON_ERROR and *_CHECK_COMPILER_ARRAY_SIZE_NEGATIVE to *-compilers.m4 along with other *_CHECK_COMPILER_*

the socklen_t issue on this platform.

HAVE_IOCTL_SIOCGIFADDR since it's needed to use SIOCGIFADDR and Watcom C
doesn't currently define it.

upcomming work to support the broadest range of Autoconf versions

used, and define HAVE_IOCTL_SIOCGIFADDR if appropriate.

checks for inet_ntoa_r() inet_ntop() and inet_pton()