it to a full path using the user's path, before setting the path used by
the script. If it can't be found, try using the standard path used by the
script to find it (/bin:/usr/bin:/usr/ucb), and if it still can't be
found, exit. If user hasn't set CC, try to use cc, again searched for
via their path, then via the standard path.
Also, clean up regexps used to post-process ${CC} -M output, so that
they work on a wider variety of compilers.