Cleanup: use 2 space indentation for CMake

CMakeLists.txt

build_files/build_environment/CMakeLists.txt

build_files/build_environment/cmake/alembic.cmake

build_files/build_environment/cmake/blendthumb.cmake

build_files/build_environment/cmake/blosc.cmake

build_files/build_environment/cmake/boost.cmake

build_files/build_environment/cmake/bzip2.cmake

build_files/build_environment/cmake/clang.cmake

build_files/build_environment/cmake/clew.cmake

build_files/build_environment/cmake/cuew.cmake

build_files/build_environment/cmake/embree.cmake

build_files/build_environment/cmake/faad.cmake

build_files/build_environment/cmake/ffi.cmake

build_files/build_environment/cmake/ffmpeg.cmake

build_files/build_environment/cmake/fftw.cmake

build_files/build_environment/cmake/flac.cmake

build_files/build_environment/cmake/flexbison.cmake

build_files/build_environment/cmake/freeglut.cmake

build_files/build_environment/cmake/freetype.cmake

build_files/build_environment/cmake/glew.cmake