Use PROJECT_SOURCE_DIR in place of CMAKE_SOURCE_DIR
and set BOOST_COMPONENTS within the PARENT_SCOPE
to be friendly to CMake projects that add_subdirectory(VexCL).

See also:
* https://github.com/clMathLibraries/clBLAS/pull/46
* http://www.cmake.org/cmake/help/v3.0/variable/PROJECT_SOURCE_DIR.html
* http://www.cmake.org/cmake/help/v3.0/command/set.html