Make configure.ac work with autoconf 2.73

Make configure.ac work with autoconf 2.73

autoreconf requires "AC_INIT" to appear in configure.ac.  Prior to
autoconf 2.73, our indirect invocation via K5_AC_INIT() was
sufficient, but now the regular expression match checks for word
boundaries.  Get rid of K5_AC_INIT(), moving the version extraction
code from aclocal.m4 to a new file version.m4.  In the moved
versioning code, use m4_fatal() to simplify error handling.

[ghudson@mit.edu: made the m4_fatal() change; rewrote commit message]

ticket: 9202 (new)