c-common.h (enum cxx_dialect): Add cxx_unset.

c-common.h (enum cxx_dialect): Add cxx_unset.

gcc/c-family/
	* c-common.h (enum cxx_dialect): Add cxx_unset.
	* c-common.c (cxx_dialect): Initialize to cxx_unset.
	* c-opts.c (c_common_post_options): Set C++ dialect to C++98 if unset.
gcc/testsuite/
	* lib/target-supports.exp (cxx_default): New global.
	(check_effective_target_c++11_only)
	(check_effective_target_c++14_only)
	(check_effective_target_c++98_only)
	(check_effective_target_c++1z_only): Check it.

From-SVN: r222977