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.

gcc/testsuite/ChangeLog

+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.
+
 2020-04-24  Andrew Stubbs  <ams@codesourcery.com>
 
 	* lib/target-supports.exp (available_vector_sizes): Add amdgcn.

gcc/testsuite/g++.dg/compat/struct-layout-1.exp

@@ -142,7 +142,19 @@ if { $status == 0 } then {
     file delete -force $tstobjdir
     file mkdir $tstobjdir
     set generator_args "-s $srcdir/$subdir -d $tstobjdir"
-    if { $use_alt == 0 } then {
+    set test_cxx14_vs_cxx17 1
+    if { $use_alt != 0 } then {
+	compat-use-alt-compiler
+	if { [check_no_compiler_messages_nocache compat_alt_has_cxx14 object {
+		int dummy; } "-std=c++14"] == 0 } {
+	    set test_cxx14_vs_cxx17 0
+	} elseif { [check_no_compiler_messages_nocache compat_alt_has_cxx17 object {
+		int dummy; } "-std=c++17"] == 0 } {
+	    set test_cxx14_vs_cxx17 0
+	}
+	compat-use-tst-compiler
+    }
+    if { $test_cxx14_vs_cxx17 != 0 } then {
 	set generator_args "$generator_args -c"
     }
     if [info exists env(RUN_ALL_COMPAT_TESTS) ] then {