aclocal: Fix Python and Perl checking macros.

  * Both were grievously broken, trying to run binaries which just
    didn't exist.  Check existence (and executability) of binaries
    before actually running them.

  * Python version checking had m4 quoting problems.  Fix them by
    liberal application of changequote.

  * Both failed to quote the proposed binary names properly.  This
    probably wasn't a problem, but I wouldn't put money on it.