Added DESTDIR support for staged installs

This pull request adds `DESTDIR` support for packages built with the cmake macros in `UseOROCOS-RTT.cmake` for staged installations (https://www.gnu.org/prep/standards/html_node/DESTDIR.html).

* `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`

The main change that also affects non-staged builds is the removal of the `INSTALL_RPATH_USE_BUILD_PATH`. Instead, all link directories from packages that have been found by `orocos_use_package()` and the link directories of the RTT core libraries are prepended to the the `INSTALL_RPATH` setting.

I only tested this patch in Ubuntu 12.04 and we would probably need further patches for other CMake versions or for MacOS. It should be considered as work in progress.