#1 Improved autoconf use

[Tim Van Holder]

Might as well add this to have a reminder for myself.

Right now, autoconf is only used for basic
configuration; in time, all sources should use #ifdef
HAVE_FOO where appropriate if using the potentially
unportable function 'foo' (similarly for headers).
configure.ac already has checks for several of these,
but they're currently disabled; since aren't used
anyway, they'd just slow down the configure run.