Mofidy logic in CMakeTests.txt files so that Linux build settings do
not override previously set APPLE build settings.  Both APPLE and UNIX
are defined in a OSX build.

Really, these CXX flags should be set in the top-level CMakeLists.txt
file, since they almost certainly will be consistent across the
project.
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.
ignore arnold files

You should get the files for arnold from SolidAngle
Make ARNOLD_PATH in cmake lists actually a path