build: Remove obsolete reference to 'GUIX_ASSERT_LIBGCRYPT_USABLE'.
Reported by Mark H Weaver <mhw@netris.org>.
Fixes <https://bugs.gnu.org/33023>.
* configure.ac: Remove use of 'GUIX_ASSERT_LIBGCRYPT_USABLE'.
Add "^GUIX_" to the forbidden M4 patterns.