Use relative paths

This brings some changes from @chadaustin upstream from imvu/master.

These changes permit using relative paths for things, which is required in some environments, like when mixing native and cygwin/mingw binaries on Windows.

I'm hopeful that the change that I made in 877c3ce27698d3af01806cb569c5f905f508c54b doesn't break things on the IMVU side, but it was required for building the relooper in the main repository.