Removes the CMP0042 policy setting of OLD which is now deprecated. Sets CMAKE_MACOSX_RPATH at the top level. Sets CMP0068 to NEW since we comply with the changes there. Removes the old INSTALL_NAME_DIR setting on libdynamorio.
I tried using @rpath or $ORIGIN for Mac explicit rpaths but both failed (@rpath is supposed to work) so I left @loader_path in place.
Cleans up some other OSX pieces such as the warning about 64-bit not being supported, and the max kernel version.
Fixes #1375 (closed)