* Update to libmongoc 1.24.3
* Define constants for POSIX features and extensions used by libmongoc 1.24
This removes old logic to not define _DEFAULT_SOURCE on Windows (technically MinGW or Cygwin). It's not clear whether that was ever necessary; it was introduced in ab44b0cbeb based on upstream work in mongodb/mongo-c-driver@9d2d8b157c, but libmongoc did not utilize the same logic.
In any event, these constants are now defined unconditionally for libmongoc since mongodb/mongo-c-driver@c5206511cd, so this change should restore parity between PHPC's autoconf and libmongoc's CMake configurations.
* Remove obsolete conditional defines for glibc 2.19
It's not clear what purpose this originally served. It dates back to mongodb/mongo-c-driver@50f701ce6a, but PHPC never incorporated the defines into its own CPPFLAGS.
The recent additions to PlatformFlags.m4 should be sufficient.
GitHub's file finder filters out "build/" directories, among others, which can make it difficult to quickly access these files. Also, the "build/" directory doesn't provide much value here, since it only contains "autotools/".
