Define _GNU_SOURCE, required for O_CLOEXEC on uClibc

When building sysklogd on a uClibc system we must define _GNU_SOURCE to
get O_CLOEXEC.

Since _GNU_SOURCE is also required for asprintf() with GLIBC, as used by
the pidfile() replacement function, we drop it from there and rely on
AM_CPPFLAGS for all sources.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

lib/pidfile.c

@@ -31,7 +31,6 @@
  * POSSIBILITY OF SUCH DAMAGE.
  */
 
-#define _GNU_SOURCE		/* Needed with GLIBC to get asprintf() */
 #include <sys/stat.h>		/* utimensat() */
 #include <sys/time.h>		/* utimensat() on *BSD */
 #include <sys/types.h>