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.

configure.ac

@@ -88,6 +88,7 @@ dnl Make sure they are available.
 m4_pattern_forbid([PKG_CHECK_MODULES])
 m4_pattern_forbid([GUILE_MODULE_AVAILABLE])
 m4_pattern_forbid([^GUILE_P$])
+m4_pattern_forbid([^GUIX_])
 
 dnl Search for 'guile' and 'guild'.  This macro defines
 dnl 'GUILE_EFFECTIVE_VERSION'.
@@ -232,8 +233,6 @@ AC_SUBST([LIBGCRYPT])
 AC_SUBST([LIBGCRYPT_PREFIX])
 AC_SUBST([LIBGCRYPT_LIBDIR])
 
-GUIX_ASSERT_LIBGCRYPT_USABLE
-
 dnl Library name of zlib suitable for 'dynamic-link'.
 GUIX_LIBZ_LIBDIR([libz_libdir])
 if test "x$libz_libdir" = "x"; then