"build_clean.sh" is used to generate a distribution tarball. The tarball
contains pregenerated man pages with default values for paths, which in
turn depend on the configure options when creating the tarball.

Previously, the man page would have paths like "usr/local/etc/NetworkManager/...",
which doesn't seem the best choice for a default man page.

Explicitly set the installation paths.

Also, --disable-dependency-tracking in this mode. It may speed up the
build.