fix uchar.h for Clang

Clang defines itself as GCC 4.2.1 and triggers check for GCC lower than
4.4.0.\
This patch adds additional !defined(__clang__) check

(cherry picked from commit dd9c402)

mingw-w64-headers/crt/uchar.h

@@ -30,7 +30,7 @@
 /* Remember that g++ >= 4.4 defines these types only in c++0x mode */
 #if !(defined(__cplusplus) && defined(__GXX_EXPERIMENTAL_CXX0X__)) ||	\
     !defined(__GNUC__) ||						\
-    (__GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 4))
+    (!defined(__clang__) && (__GNUC__ < 4 || (__GNUC__ == 4 &&	__GNUC_MINOR__ < 4)))
 typedef uint_least16_t char16_t;
 typedef uint_least32_t char32_t;
 #endif