Continuing to sanitize the build for cases where
LumiverseCore_INCLUDE_ARNOLD is OFF.  There is a lot of breakage from
code not checking for whether USE_ARNOLD is defined.

All Arnold build setting logic is now in the top-level CMakeLists.txt
file since it needs to apply both to LumiverseCore, and all the tests
and demo directories.

Still not building on OSX due to a few more C++-11 issues that Clang
seems to be stricter on.