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.

(reopened from #50)