Use relative paths in __init__.py

[ryanmrichard]

Right now the __init__.py files generated by the CMake API use absolute paths for the headers and libraries which are set to the build directory. For installation purposes, these paths should be relative to an installation root (said root is usually a staging directory while building, and the actual installation root after install).

Off the top of my head I'm not sure where Python bindings are usually installed in a typical GNU build (lib maybe?), but I'm sure there's a precedent.

Since CMaize currently does not support installation, this is not a high-priority issue.

[wadejong]

This should be something we can simplify. But, this means we need to agree what the install looks like. We can install Python packages in the same location as the headers and binaries, and require PYTHONPATH to be set. If you want the python packages in the normal python location, we need to make sure both are synchronized.