util: Prevent strcasecmp macro redefinion.

MinGW headers already define it.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Acked-by: Eric Engestrom <eric@engestrom.ch>

src/util/u_string.h

@@ -110,7 +110,10 @@ util_asprintf(char **str, const char *fmt, ...)
    return ret;
 }
 
+#ifndef strcasecmp
 #define strcasecmp stricmp
+#endif
+
 #define strdup _strdup
 
 #endif