testsuite: C++14 vs. C++17 struct-layout-1.exp testing with ALT_CXX_UNDER_TEST [PR94383]

> I haven't added (yet) checks if the alternate compiler does support these
> options (I think that can be done incrementally), so for now this testing is
> done only if the alternate compiler is not used.

This patch does that, so now when testing against not too old compiler
it can do the -std=c++14 vs. -std=c++17 testing also between under test and
alt compilers.

2020-04-24  Jakub Jelinek  <jakub@redhat.com>

	PR c++/94383
	* g++.dg/compat/struct-layout-1.exp: Use the -std=c++14 vs. -std=c++17
	ABI compatibility testing even with ALT_CXX_UNDER_TEST, as long as
	that compiler accepts -std=c++14 and -std=c++17 options.