On Mac, set DYLD_FRAMEWORK_PATH in addition to DYLD_LIBRARY_PATH

This change updates the top level CMakeLists.txt to capture the
DYLD_FRAMEWORK_PATH environment variable, too.  I also changed the
doc strings for the cache variables and added calls to mark_as_advanced.
Merge pull request #326 from patmarion/fix-systemWCMakeEnv-for-mac

On Mac, set DYLD_FRAMEWORK_PATH in addition to DYLD_LIBRARY_PATH