cmake/useorocos: added DESTDIR support for staged installs

* DESTDIR is prepended to all include and link directories found by pkg-config
* OROCOS-RTT_LIBRARY_DIRS variable added to OROCOS-RTT cmake config for RTT core libraries, plugins and typekits
* Removed INSTALL_RPATH_USE_BUILD_PATH flag
* INSTALL_RPATH is constructed from USE_OROCOS_LIBRARY_DIRS_WITHOUT_DESTDIR and OROCOS-RTT_LIBRARY_DIRS

See also: https://www.gnu.org/prep/standards/html_node/DESTDIR.html

Signed-off-by: Johannes Meyer <johannes@intermodalics.eu>
Merge pull request #51 from orocos-toolchain/destdir-support

Added DESTDIR support for staged installs