Log Message:
Beat some sense in, to make it easier to debug build failures.

   - Generate all files into the current (build) directory, not /tmp.
   - Do not delete any of the temporary files, to allow analysis after
     build failure.
   - Log the command lines executed.
   - Warn if scrubbing the Imakefile to alert the user to check if that
     broke it.

This currently does not capture the munging imake does on the output
from cpp, as that is disgustingly done in place and changing that
would require more extensive hacking. However, it's now readily
possible to rerun the cpp command directly for diagnostic purposes;
the previously-secret file that imake uses to start the cpp invocation
is now left in .imake.start.c.

PKGREVISION -> 1.