Fix test 86658 in Debug mode

[Jonathan Wakely]

On 18/08/18 22:09 +0200, François Dumont wrote:
>This test fails when using 'make check-debug' because it redefines 
>_GLIBCXX_DEBUG. Using dg-options fixes this problem.

Changing it to "#define _GLIBCXX_DEBUG 1" would also have fixed it,
because that would be the same value as -D_GLIBCXX_DEBUG sets, and a
redefinition to the same value doesn't warn.

Using dg-options is cleaner though, thanks.