[libcxx] Remove installation rules on Darwin when it would overwrite the system installation.

Summary:
On Mac OS X overwriting `/usr/lib/libc++.dylib` can cause your computer to fail to boot. This patch tries to make it harder to do that accidentally. 

If `CMAKE_SYSTEM_NAME` is `Darwin` and `CMAKE_INSTALL_PREFIX` is `/usr` don't generate installation rules unless the user explicitly provides `LIBCXX_OVERRIDE_DARWIN_INSTALL=ON`. Note that `CMAKE_INSTALL_PREFIX` is always absolute so we don't need to worry about things like `/usr/../usr`.

Reviewers: mclow.lists, beanz, jroelofs

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D12209

llvm-svn: 246070