Fixes for the Wno-dev warnings

Use VERBATIM in custom commands to fix quoting, to make CMake happy with the Python command being issued.  This change should be compatible with both CMake 2.6 and 2.8; see "add_custom_command", VERBATIM in both 2.6 and 2.8 docs:
< http://www.cmake.org/cmake/help/v2.8.12/cmake.html#command:add_custom_command >
< http://www.cmake.org/cmake/help/cmake2.6docs.html#command:add_custom_command >
This change works for me on OSX, but should be OS agnostic.