ax_enable_builddir and ax_prefix_config_h updated

[Guido Draheim]

ax_enable_builddir - a typo with bad side effects
ax_prefix_config_h - rewritten to use shorthands for variables

I've been giving it a few test rounds in the last days,
I'll check about it a bit further and put out a new
release archive at sourceforge later today. I am
worried that there is not much traffic on the mailinglist
or the archive repositories themselves, perhaps it helps
to make a bit propaganda to attract people to find out
more about the ac-archive idea.

-- cheers, guido