Fixed cmake source/build dir references to be able to build STP as subproject

By using PROJECT_BINARY_DIR instead of CMAKE_BINARY_DIR, stp can be built
as a CMAKE subproject to other projects.

Signed-off-by: Jonas Zaddach <zaddach@eurecom.fr>
Merge pull request #141 from zaddach/jonas/build-as-cmake-subproject

Fixed cmake source/build dir references to be able to build STP as subpr...