testsuite, Objective-c++ : Fix GNU API tests to work with NeXT where possible.

testsuite, Objective-c++ : Fix GNU API tests to work with NeXT where possible.

The version 2 GNU Objective C API is mostly compatible with the NeXT one.
However, there are a few testsuite tweaks needed (and one test fails for NeXT
without considerable increase in complexity).

gcc/testsuite/ChangeLog:

	* obj-c++.dg/gnu-api-2-class-meta.mm: Add a flag to cause NeXT
	headers to expose prototypes for messaging.
	* obj-c++.dg/gnu-api-2-class.mm: Likewise.
	* obj-c++.dg/gnu-api-2-objc.mm: Skip this because of the extra
	protocols pulled in by system headers.