Linux Makefile "uninstall" target doesn't remove directories

The uninstall and uninstalldebug targets simply call

xargs rm < install_manifest.txt

to delete all of the files created during installation, as listed in the file "install_manifest.txt".

However, this does not remove the directories containing those files. The directories were created as part of the install process so they should be removed now as part of the uninstall process.

This is particularly a problem if the user overrides the PREFIX or SUFFIX makefile variables, because empty directory trees could be left all over the system.