install: Handle the case where parts of install tree are not user owned
@walters
Submitted by Colin Walters Link to original bug (#655537)
Description
In some cases like dbus for historical reasons we've recommended linking bits of the install tree to the root-owned system tree.
Currently process_install_files will throw an exception in this case. Before, we blindly ran 'make install' which would throw a subprocess error, and the user could (potentially) continue the build, even though it's sort of undefined what files would be installed in that case.
We might be able to detect the case where we will try to create or overwrite a file in a root-owned directory, and skip it.