Disable use of [[deprecated]] with gcc 5

This gcc version claims support for this attribute, but actually doesn't
support it and returns unclear errors when it is used, e.g.

	$ echo '[[deprecated]] int x;' | g++-5 -fsyntax-only -x c++ -
	<stdin>:1:1: error: expected unqualified-id before ‘[’ token

so disable its use with gcc < 6.

include/wx/defs.h

@@ -585,7 +585,10 @@ typedef short int WXTYPE;
 
 #if defined(__has_cpp_attribute)
     #if __has_cpp_attribute(deprecated)
-        #define wxHAS_DEPRECATED_ATTR
+        /* gcc 5 claims to support this attribute, but actually doesn't */
+        #if !defined(__GNUC__) || wxCHECK_GCC_VERSION(6, 0)
+            #define wxHAS_DEPRECATED_ATTR
+        #endif
     #endif
 #endif