scripts: Fix fortify checks if compiler does not support _FORTIFY_SOURCE=3

The 30379efa added _FORTIFY_SOURCE checks without check if compiler
does support all used fortify levels.  This patch fixes it by first
checking at configure time the maximum support fortify level and using
it instead of a pre-defined one.

Checked on x86_64 with gcc 11, 12, and 13.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
Tested-by: Florian Weimer <fweimer@redhat.com>