configure.ac: Enable assert(3) by default.

Revert 167e542b that disabled assert(3) by default after private email
discussion with David Levine.  I suggested it being enabled by default
in git so those running nmh from git were testing them for us, with it
being disabled as part of the release process so packagers continue with
it disabled.  David pointed out that would make the release
intentionally different from what had been long tested so the default is
now enabled assertions everywhere unless disabled by a packager.