* configure.in (have_pygtk): Add test for Pygtk.

* acconfig.h: Add WITH_PYGTK

* plugins/python/python-loader.c: Include pygobject.h if Pygtk is
installed.
(initialize_python_if_needed): Add init_pygobject if Pygtk is
installed.
(gnumeric_plugin_loader_python_func_file_probe): Replace file name
argument with GsfInput. Try to wrap GsfInput in Python object if
Pygtk is installed. Otherwise give up with warning.
(gnumeric_plugin_loader_python_func_file_open): Ditto.

* plugins/python/Makefile.am: Add PYGTK_CFLAGS to includes.