C++ PATCH for decomp31.C test

[Marek Polacek]

Running make check-c++ RUNTESTFLAGS=dg.exp=decomp31.C will yield
# of unsupported tests          3
because the test (as the only one in cpp1z/) uses
"dg-do compile { target c++17 }" which doesn't work (yet?).  This patch
makes it use explicit dg-options as in other tests, so now we get
# of expected passes            1

Tested on x86_64-linux, ok for trunk?

2018-10-11  Marek Polacek  <pola...@redhat.com>

        * g++.dg/cpp1z/decomp31.C: Use explicit dg-options.

--- gcc/testsuite/g++.dg/cpp1z/decomp31.C
+++ gcc/testsuite/g++.dg/cpp1z/decomp31.C
@@ -1,5 +1,6 @@
 // PR c++/81888
-// { dg-do compile { target c++17 } }
+// { dg-do compile }
+// { dg-options "-std=c++17" }
 
 struct S {
   bool s = true;