configure.ac: Detect wchar_t using wchar.h header

The wchar_t type is defined in `wchar.h` and if this header is not
included by autoconf the detection of the type will fail.  This type is
required by `unicharset_extractor` to autogenerate the character
properties.

This problem was detected when running under Fedora 21.

Author: johnteslade

configure.ac

@@ -314,7 +314,7 @@ AC_SYS_LARGEFILE
 # Checks for typedefs, structures, and compiler characteristics.
 # ----------------------------------------
 
-AC_CHECK_TYPES(wchar_t)
+AC_CHECK_TYPES(wchar_t,,,[#include "wchar.h"])
 AC_CHECK_TYPES(long long int)
 AC_CHECK_TYPES(mbstate_t,,,[#include "wchar.h"])