g_check_setuid: implement using getauxval(AT_SECURE) with glibc

See commit 4c2928a544 for why checking AT_SECURE is preferable compared
to UID checks as currently done in the fallback case.

getauxval() was added with glibc 2.16

While glibc <2.19 didn't provide a way to differentiate a 0 return value from an error,
passing AT_SECURE should always succeed according to
https://sourceware.org/ml/libc-alpha/2014-07/msg00407.html
I've added an errno check anyway, to be on the safe side.

meson.build

@@ -229,6 +229,7 @@ headers = [
   'stdlib.h',
   'string.h',
   'strings.h',
+  'sys/auxv.h',
   'sys/event.h',
   'sys/filio.h',
   'sys/inotify.h',