define _GNU_SOURCE on the command line

that way it is already set in configure and can thus be used by tests.

configure.in

@@ -9,6 +9,8 @@ if test "$GCC" = yes; then
     CFLAGS="$CFLAGS -pipe -W -Wall -Wshadow -Wstrict-prototypes"
 fi
 
+CPPFLAGS="$CPPFLAGS -D_GNU_SOURCE"
+
 AC_CHECK_HEADERS(sys/poll.h sys/select.h)
 AC_CHECK_FUNCS(vasprintf)
 

src/compat/isync.h

@@ -18,8 +18,6 @@
  *  Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA
  */
 
-#define _GNU_SOURCE
-
 #include <config.h>
 
 #include <sys/types.h>

src/compat/main.c

@@ -35,7 +35,6 @@
 #include <dirent.h>
 
 #ifdef HAVE_GETOPT_LONG
-# define _GNU_SOURCE
 # include <getopt.h>
 struct option Opts[] = {
 	{"write", 0, NULL, 'w' },

src/isync.h

@@ -21,8 +21,6 @@
  * despite that library's more restrictive license.
  */
 
-#define _GNU_SOURCE
-
 #include <config.h>
 
 #include <sys/types.h>