Make path to cmake modules relative to CMakeLists.txt

There was a problem when including the PCL as a dependency using the add_subdirectory(...) command. The cmake modules would not be found, since one would not invoke the CMakeLists.txt from the PCL root. Here is the cmake documentation (from http://www.cmake.org/Wiki/CMake_Useful_Variables):

*CMAKE_SOURCE_DIR*
    this is the directory, from which cmake was started, i.e. the top level source directory

*CMAKE_CURRENT_SOURCE_DIR*
    this is the directory where the currently processed CMakeLists.txt is located in
Merge pull request #504 from grtlr/cmake

Make path to cmake modules relative to CMakeLists.txt