Stopped using python-config for build configuration.

This is for a number of reasons :

- python-config returns paths hardcoded to include the install location it was built with. This meant that we couldn't provide a packaged set of binary dependencies, making building Gaffer much harder than it needed to be.
- We were having to patch the python-config output for the link flags on mac anyway.
- It didn't work for the Image Engine build - we were just ignoring the errors and getting away with it somehow.