Fix test 86658 in Debug mode

[François Dumont]

This test fails when using 'make check-debug' because it redefines 
_GLIBCXX_DEBUG. Using dg-options fixes this problem.

Committed as trivial.
François

diff --git a/libstdc++-v3/testsuite/25_algorithms/copy/86658.cc b/libstdc++-v3/testsuite/25_algorithms/copy/86658.cc
index 600747a..dd4f7ba 100644
--- a/libstdc++-v3/testsuite/25_algorithms/copy/86658.cc
+++ b/libstdc++-v3/testsuite/25_algorithms/copy/86658.cc
@@ -15,9 +15,9 @@
 // with this library; see the file COPYING3.  If not see
 // <http://www.gnu.org/licenses/>.
 
+// { dg-options "-D_GLIBCXX_DEBUG" }
 // { dg-do run }
 
-#define _GLIBCXX_DEBUG
 #include <algorithm>
 #include <vector>