Rename configure.in to configure.ac - the former has been deprecated
since 2001.
Change final message from configure to something less confusing.
Rename HAVE_LIBCRYPT to HAVE_CRYPT for several reasons, mainly because
it's what core John uses. One other reason is that we no longer really
depend on the library per se, but the function (it might be present in
libc).

Also revert a change that broke build on OSX and likely others.