CMake should parse LD_LIBRARY_PATH and PYTHONPATH

Using bake, the click, openflow, and brite libraries are installed in bake/build, a non-standard place. In the past, we have asked users who use bake to extend LD_LIBRARY_PATH to include bake/build/lib so that Waf could find these libraries. Current CMake as of commit 33f2ee84 does not do so.

Similarly, PYTHONPATH should be checked if Python is enabled (which is related to !833 (merged)).