gutils: Check whether getauxval function exists

FreeBSD 12 adds a new header, sys/auxv.h, to declare a function, elf_aux_info,
for public use, which was considered an internal function in previous releases.
This new function provides similar functionality with glibc getauxval, which is
also declared in the same header, but their interfaces are not compatible. Since
the only usage of sys/auxv.h is in g_check_setuid and FreeBSD already has
issetugid to provide the required functionality, we fixes the compilation error
by adding a check for getauxval function to prevent g_check_setuid from calling
getauxval when sys/auxv.h is found but getauxval is not available.

https://reviews.freebsd.org/D12743
https://reviews.freebsd.org/rS324815

glib/gutils.c

@@ -2503,7 +2503,7 @@ const gchar *g_get_tmp_dir_utf8 (void) { return g_get_tmp_dir (); }
 gboolean
 g_check_setuid (void)
 {
-#if defined(HAVE_SYS_AUXV_H)
+#if defined(HAVE_SYS_AUXV_H) && defined(HAVE_GETAUXVAL) && defined(AT_SECURE)
   unsigned long value;
   int errsv;