bpo-42692: fix __builtin_available check on older compilers (GH-23873) (GH-24090)

A compiler that doesn't define `__has_builtin` will error out when it is
used on the same line as the check for it.

Automerge-Triggered-By: GH:ronaldoussoren
(cherry picked from commit df21f50)

Co-authored-by: Joshua Root <[email protected]>

Author: miss-islington

Misc/NEWS.d/next/Build/2021-01-04-05-07-30.bpo-42692.OO11SN.rst

@@ -0,0 +1 @@
+Fix __builtin_available check on older compilers. Patch by Joshua Root.

Modules/posixmodule.c

@@ -56,7 +56,13 @@
  */
 #if defined(__APPLE__)
 
-#if defined(__has_builtin) && __has_builtin(__builtin_available)
+#if defined(__has_builtin)
+#if __has_builtin(__builtin_available)
+#define HAVE_BUILTIN_AVAILABLE 1
+#endif
+#endif
+
+#ifdef HAVE_BUILTIN_AVAILABLE
 #  define HAVE_FSTATAT_RUNTIME __builtin_available(macOS 10.10, iOS 8.0, *)
 #  define HAVE_FACCESSAT_RUNTIME __builtin_available(macOS 10.10, iOS 8.0, *)
 #  define HAVE_FCHMODAT_RUNTIME __builtin_available(macOS 10.10, iOS 8.0, *)