#5 configure's up-to-date check is too slow

[Tyson Dowd]

The check for up-to-date C files that configure runs
is very slow, particularly under cygwin.

We should rewrite it so that it doesn't create as many
processes. There is some room for improvement in the
(very common) case of no files being misconfigured.
It would be fine for it to use a slow method to
determine which files actually need to be regenerated
as this happens very rarely.