posix_spawn-internal: Fix a warning

[Bruno Haible]

Compiling GNU gettext on CentOS 5 (gcc 4.1.2), I see this warning:

gettext-tools/gnulib-lib/spawni.c:861: warning: ignoring #pragma GCC diagnostic 
[-Wunknown-pragmas]

This particular pragma is only supported without warning for gcc ≥ 4.2.


2023-06-05  Bruno Haible  <bruno@clisp.org>

        posix_spawn-internal: Fix a warning (regression 2022-11-20).
        * lib/spawni.c: Don't use this particular #pragma for gcc < 4.2.

diff --git a/lib/spawni.c b/lib/spawni.c
index cc9511fdd8..84d1fb27f6 100644
--- a/lib/spawni.c
+++ b/lib/spawni.c
@@ -857,7 +857,7 @@ __spawni (pid_t *pid, const char *prog_filename,
 /* The warning "warning: 'vfork' is deprecated: Use posix_spawn or fork" seen
    on macOS 12 is pointless, as we use vfork only when it is safe or when the
    user has explicitly requested it.  Silence this warning.  */
-#if __GNUC__ >= 3
+#if _GL_GNUC_PREREQ (4, 2)
 # pragma GCC diagnostic ignored "-Wdeprecated-declarations"
 #endif