fix compile of libs/vfork with slightly older(?) glibc headers

libs/vfork/exec_wrapper.c

@@ -99,3 +99,4 @@ int main(int argc, char *argv[]) {
 	exit(-1);
 	return -1;
 }
+

libs/vfork/wscript

@@ -24,5 +24,5 @@ def build(bld):
     obj.install_path = os.path.join(bld.env['LIBDIR'], 'ardour3/vfork')
     obj.defines = [
             '_POSIX_SOURCE',
-            '_XOPEN_SOURCE=700',
+            '_XOPEN_SOURCE=500',
     ]