Don't define __USE_GNU, thats a glibc-internal macro, and
don't use SOL_SOCKET when not including sys/socket.h.
Maybe this file should be called glinuxcredentialsmessage.c...
Bug #618730
Things compile and the test-suite passes. Still need to hook up
gio.symbols and docs. There are still a bunch of TODOs left in the
sources that needs to be addressed.
Signed-off-by: David Zeuthen <davidz@redhat.com>