The function AC_ARG_ENABLE provided by autoconf is a very bare-bones function, which needs a lot of boilerplate code to do the right thing. We have duplicated this code all over the place, often with bugs or omissions of functionality.

This patch introduces a new UTIL_ARG_ENABLE which sets up a best-practices framework for handling AC_ARG_ENABLE, and allows users to just call this and be sure we do the right thing.

I have also tried to keep the difference introduced in the JVM features patch between "available" and "enabled", so it's clear if a feature is not enabled because it is not possible to enable, or just because it is not the default.

This cleanup highlights the major differences in how we handle this kind of flags, for instance if we set the flag value to "true" or "yes" in spec.gmk. I have opted not to touch any of that in this patch, but I feel a follow-up cleaning might be warranted.