allow autom4te to accept -D and -U as m4 does

[Matěj Týč]

Hello,
the subject says it all, m4 allows you to define and/or undefine macros,
so if one uses it e.g. in Makefiles, it is easy to alter the output.

However, m4 is a real pain to use, m4sugar is so much better. But
autom4te, the convenient m4sugar "frontend", doesn't accept these
options, which I would like to change. I can elaborate more on use cases
if you are interested, but as this change is minor and
backwards-compatible, I hope that there will not be much resistance in
accepting it :-)

The patch is attached.
There are, however, some things I am not sure about - namely caching and
freezing. I don't have insight into them, but I have suspicion that they
may interfere with the new define/undefine stuff.

autom4te.diff
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