Try to fix macOS build using cmake after xlocale changes

Commit bc13119494 removed the inclusion of
xlocale.h because it is not (and never was) needed under Linux with
glibc, but it is still needed under macOS, so this (silently) disabled
wxXLocale support under Mac when using configure and broke the build
when using cmake.

Fix both problems by using xlocale.h only if it's available, both in
configure and in cmake.

include/wx/xlocale.h

@@ -40,6 +40,11 @@
         typedef _locale_t wxXLocale_t;
         #define wxXLOCALE_IDENT(name) _ ## name
     #elif defined(HAVE_LOCALE_T)
+        // Some systems (notably macOS) require including a separate header for
+        // locale_t and related functions.
+        #ifdef HAVE_XLOCALE_H
+            #include <xlocale.h>
+        #endif
         #include <locale.h>
         #include <ctype.h>
         #include <stdlib.h>