Define our desires for POSIX and X/OPEN API features properly.

Define our desires for POSIX and X/OPEN API features properly.

Based on a post on the gcc-help mailing list and some subsequent reading,
we can increase our portability to various platforms by directly defining
the POSIX and X/OPEN API feature sets we wish to have available. This patch
does that, and also includes a double-check to ensure that the system
we are compiling on can actually provide the requested feature sets.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@212463 65c4cc65-6c06-0410-ace0-fbb531ad65f3