shared: fix _POSIX_C_SOURCE not being defined for nm-errno.c

This is the case with musl.

Based-on-patch-by: Rasmus Thomsen <oss@cogitri.dev>

shared/nm-glib-aux/nm-errno.c

@@ -106,7 +106,7 @@ nm_strerror_native_r(int errsv, char *buf, gsize buf_size)
     nm_assert(buf);
     nm_assert(buf_size > 0);
 
-#if (_POSIX_C_SOURCE >= 200112L) && !_GNU_SOURCE
+#if (!defined(__GLIBC__) && !defined(__UCLIBC__)) || ((_POSIX_C_SOURCE >= 200112L) && !_GNU_SOURCE)
     /* XSI-compliant */
     {
         int errno_saved = errno;