I made 3 minor adjustments to the cmake build scripts.
added ARNOLD_HOME as a valid env variable. this is the variable that solid angle has been using in their internal scripts since I first started working on mtoa, so it'd be good to encourage its adoption
added support for CMAKE_PREFIX_PATH: this is cmake's official variable for setting the install location. To keep backward compatibility, I used it as the default for when neither INSTALL_DIR nor INSTALL_ROOT are provided. I think this is reasonable since the old default was the build directory, and I doubt many users are relying on this behavior.
parsed ARNOLD_VERSION from ai_version.h. I don't really need this, but I thought I'd add it. You could consider removing ARNOLD_VERSION as an explicit input altogether, since there is the possibility of a mismatch between what's provided by the user and what's actually found.