Allows absolute and relative paths for
- `INCLUDE_INSTALL_DIR`
- `CMAKEPACKAGE_INSTALL_DIR`
- `PKGCONFIG_INSTALL_DIR`

Type should be `PATH` not `STRING`.  Contrary to !211, these don't
seem to be made absolute if user-defined - according to the doc any
directories should use `PATH` type, which allows a file dialog
to be used via the GUI.  It also better handles file separators.

If user provides an absolute path, it will be made relative to
`CMAKE_INSTALL_PREFIX` so that the `configure_packet_config_file` will
work.

Fixes #2155 and #2269.