Use PyCapsule for py3.1, fixes #237
Merge pull request #238 from wiredfool/pycapsule

Use PyCapsule for py3.1, fixes #237
Support --disable-* switches to disable features.

For example, --disable-jpeg unconditionally disables both checks jpeg
support and checks for libjpeg.
Introduce abstraction over --disable-feature.
Introduce --enable-x and fail if it is given and x is not available.
Merge pull request #240 from mgorny/master

Support explicitly disabling features via --disable-* options