Added optional flag to gnumeric to allow python scripting

Gnumeric allows users to write macros/scripts in python.
That was unavailable in the previous version.  Gnumeric's configure
script would see pygobject missing and disable it.  With this
commit, users can do --with-pygobject and enable it.
(I'd prefer that the option was named "--with-python" but I didn't
know how to do that.)