includes do not support absolute paths

Issue Summary

It is useful for us to reference system-wide include files. For example, we would like to override the aegir makefile in our own distribution, this way:

or, with the debian package:

Neither works. The documentation says the path is "relative" but doesn't explicitely say that absolute paths are forbidden. I tried an absolute path, thinking myself very smart, but that didn't work either:

... even though that actually works on the commandline.

This needs to be clarified in the documentation or clearly allowed. I would like to see this implemented.