string: Fix tester build with fortify enable with gcc 6

When building with fortify enabled, GCC 6 issues an warning the fortify
wrapper might overflow the destination buffer.  However, GCC does not
provide a specific flag to disable the warning (the failure is tied to
-Werror).  So to avoid disable all errors, only enable the check for
GCC 7 or newer.

Checked on i686-linux-gnu.
Reviewed-by: Carlos O'Donell <carlos@redhat.com>