Replace ${CMAKE_SOURCE_DIR} with ${PROJECT_SOURCE_DIR}

[cehberlin]

Replaced ${CMAKE_SOURCE_DIR} with ${PROJECT_SOURCE_DIR}to enable embedding of octomap into other source structures, for instance through the add_subdirectory() command

[ahornung]

@cehberlin I just found that this places the compiled binaries and libs always in the sub-project lib/bin dirs (e.g. octovis in octomap/octovis/bin/octovis) no matter how the project is compiled.

When compiling only a sub-project then this should be the case, but when compiling the complete distribution, the binaries should end up in the "main" bin dir (e.g. octovis in octomap/bin/octovis).

Could you have a look and change it accordingly with a PR? There could be a switch depending on whether CMake was originally run for the octomap-distribution project.

I'm currently looking for loose ends for a soon-ish new release.

[ahornung]

This could also be achieved by placing links in the top-level bin directory, not sure if the libs should be all collected in one place anyways. This also relates to #133