Warn when absolute paths are used with build_dir.
Warn when absolute paths are used with build_dir.

This closes #2323.

scoder/cython,cython/cython,da-woods/cython,scoder/cython,cython/cython,scoder/cython,da-woods/cython,cython/cython,da-woods/cython,cython/cython,da-woods/cython,scoder/cython