[bug#74434] [PATCH] py-compile: Allow user to disable python

Hi Frederic,

    I believe this is a common practice 

Well, clearly no one had thought of it nor was it tried at any point
during the lengthy automake pretest sequence, but anyway, agreed that
there's no reason to gratuitously fail in this case.

As for disabling pythong, it doesn't seem to me that setting PYTHON=:
can really be considered equivalent to something like a --without-python
option, but since it helps in some cases, that's fine. --thanks, karl.