Use std::string::compare() unconditionally

Don't test for it in configure/CMake and just always use it.

include/wx/defs.h

@@ -315,10 +315,6 @@ typedef short int WXTYPE;
 /* for consistency with wxStatic/DynamicCast defined in wx/object.h */
 #define wxConstCast(obj, className) const_cast<className *>(obj)
 
-#ifndef HAVE_STD_STRING_COMPARE
-    #define HAVE_STD_STRING_COMPARE
-#endif
-
 #ifndef HAVE_TYPE_TRAITS
     #define HAVE_TYPE_TRAITS
 #endif