updating to autoconf 2.5

[Benjamin Kosnik]

>The last time I tried, only minor/inconsequential parts could be changed.
>The really big advances, e.g., replacing or removing the custom hacks in
>GLIBCPP_CONFIGURE, require fixes or updates in autoconf.  (I think the
>more recent releases might have them, but haven't tried.)  The worthwhile
>part IMHO will be when we can get rid of the hardcoded cross-compile crap
>(abbreviated HCCC or maybe H3C) in configure.in.

.. actually doing per-directory conditional compilation in a clean
manner would be possible if AC_OUTPUT(...) was removed and the
AC_CONFIG_FILES/AC_OUTPUT bit was used instead. Meaning, there wouldn't
be directories with empty Makefile rules in the build dir when the
subdir wasn't needed. Sigh.

I suppose there are other things, but that's the most annoying to me now.

-benjamin