one doesn't *do* anything by default; it's just there as a conduit for data
(eg. include dirs, libraries) from the user to the "build" commands.
However, it provides a couple of Autoconf-ish methods ('try_compile()',
'try_link()', 'try_run()') that derived, per-distribution "config" commands
can use to poke around the target system and see what's available.

Initial experimenst with mxDateTime indicate that higher-level methods are
necessary: analogs of Autoconf's AC_CHECK_HEADER, AC_CHECK_LIB will be
needed too (and that's just to probe the C/C++ system: how to probe the
Python system is wide open, and someday we'll have to worry about probing a
Java system too).