build: enable CMake policy 0077

Enable CMake policy 77. This alters the behavior of option. The old behavior
would remove the value of the option from the cache and create a new one. The
new behavior does not create the variable if it is defined already. This ensures
that subsequent reconfigures will behave identically. This seems better than the
setting of OLD - the desire is to ensure that it is set to OLD or NEW.

llvm-svn: 361307