Let the perl program be configurable via the PERL environment variable

This is useful if, for some reason, one wants to generate a module with a
different perl than the first one in the current PATH, or if perl is not found
is the current path and one does not want to modify PATH.

The default value 'perl' is used (as before) if PERL is not defined.
In the generated autogen script, let autoconf be configurable

Use 'autoconf' (as before), or the value of the environment variable AUTOCONF
if it is set.
[python] Fix the code generated for enum