configure.ac: Forbid AX_-prefixed patterns more selectively
We want to make autoconf fail early and with a user-comprehensible message if autoconf-archive isn't installed, rather than generating a configure script with syntax errors, or a configure script that runs successfully but doesn't do what we intended.
However, autoconf-archive doesn't actually guarantee not to use AX_-prefixed shell variable names without m4_pattern_allow'ing them (unlike Autoconf, Automake, Libtool and pkg-config, which explicitly use m4_pattern_allow for variables with AC_, AM_, LT_ and PKG_ prefixes), so it isn't safe to assume that they won't be used. In particular, recent versions of AX_CHECK_GNU_MAKE appear to be using $AX_CHECK_GNU_MAKE_HEADLINE as a shell variable.
Instead, specifically forbid the names of the finite list of macros that we actually use.