libplist @1.10: add python32 and python33 variants

[zpeeters@…]

Added python32 and python33 variants. Other python versions could easily be added as well, but have not tested.

Other changes:

• A default python variant is set, but python is not necessary for building libplist. Instead, when setting a python variant, python bindings for libplist are compiled and installed. Removed the default variant, since the python bindings are an optional extra.

• Also changed the way depends and configure.args are set, more straightforward.

• Changed variant descriptions to reflect that it's python bindings.

• python32 and python33 variants use ${frameworks_dir}/Python.framework/Versions/3.3/Headers as include dir, because the actual dir might include the python abi in the name and depends on the installation. Changed the include dirs in python26 and python27 variants to be consistent.

These changes do not address ticket ​#41948