Don't check for non-standatd xlocale.h in configure

Testing for xlocale.h was due to a misunderstanding, this header wasn't
supposed to define locale_t which is defined by locale.h itself and was
just some internal glibc header which was removed in its 2.26 release,
see

https://sourceware.org/glibc/wiki/Release/2.26#Removal_of_.27xlocale.h.27

Stop checking for it in configure and also don't always define
wxUSE_XLOCALE but only do it if the configure test succeeded.

include/wx/xlocale.h

@@ -41,7 +41,6 @@
         #define wxXLOCALE_IDENT(name) _ ## name
     #elif defined(HAVE_LOCALE_T)
         #include <locale.h>
-        #include <xlocale.h>
         #include <ctype.h>
         #include <stdlib.h>