Fix locale_t under macOS 10.10

`xlocale.h` is not available on Linux, so we can't just universally
include it.

`HAVE_XLOCALE_H` was already being tested/set in the CMake script as a
possible requirement for `wcstod_l` support, this just adds it to
`config_cmake_h.in` and uses it in `wutil.h` to gate the include.

config_cmake.h.in

@@ -156,6 +156,9 @@
 /* The size of wchar_t in bits. */
 #define WCHAR_T_BITS ${WCHAR_T_BITS}
 
+/* Define if xlocale.h is required for locale_t or wide character support */
+#cmakedefine HAVE_XLOCALE_H 1
+
 /* Enable large inode numbers on Mac OS X 10.5.  */
 #ifndef _DARWIN_USE_64_BIT_INODE
 # define _DARWIN_USE_64_BIT_INODE 1

src/wutil.h

@@ -10,6 +10,10 @@
 #include <locale.h>
 #include <string>
 
+#ifdef HAVE_XLOCALE_H
+#include <xlocale.h>
+#endif
+
 #include "common.h"
 #include "maybe.h"