Build process.c with _GNU_SOURCE (#363)

Otherwise the build will fail on glibc 2.29 and later because
posix_spawn_file_actions_addchdir_np is not declared.

Starting with glibc 2.37, posix_spawn_file_actions_addchdir_np
will be available with the default feature flags; see glibc
commit 2ff48a4025515e93d722947a9eabb114f4a65b22 ("posix: Make
posix_spawn extensions available by default").

Author: fweimer-rh

Sources/TSCclibc/process.c

@@ -1,5 +1,9 @@
 #if defined(__linux__)
 
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE /* for posix_spawn_file_actions_addchdir_np */
+#endif
+
 #include <errno.h>
 
 #include "process.h"