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.

build/cmake/setup.h.in

@@ -1257,6 +1257,9 @@
 /* Define if setpriority() is available. */
 #cmakedefine HAVE_SETPRIORITY 1
 
+/* Define if xlocale.h header file exists. */
+#cmakedefine HAVE_XLOCALE_H 1
+
 /* Define if locale_t is available */
 #cmakedefine HAVE_LOCALE_T 1