compile with -std=gnu99 and _GNU_SOURCE defined

Should enable _BSD_SOURCE, _SVID_SOURCE, _ATFILE_SOURCE,
_LARGEFILE64_SOURCE, _ISOC99_SOURCE, _XOPEN_SOURCE_EXTENDED,
_POSIX_SOURCE, _POSIX_C_SOURCE, and _XOPEN_SOURCE.

We already require features exposed by some of these defines. Lets
enable them globally to avoid problems caused by the ordering of
includes.

Change-Id: I562c936780c6b1341995281737d4378d8c9b2be7