[cmake] Set cmake policy CMP0068 to suppress warnings on OSX

Set cmake policy CMP0068=NEW, if available, and set
"CMAKE_BUILD_WITH_INSTALL_NAME_DIR=On" globally to
maintain current behavior.

This is needed to suppress warnings on OSX starting with cmake version
3.9.6.

Differential Revision: https://reviews.llvm.org/D42463

llvm-svn: 323404