[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

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@246070 91177308-0d34-0410-b5e6-96231b3b80d8

cmake/Modules/HandleLibCXXABI.cmake

@@ -41,11 +41,13 @@ macro(setup_abi_lib abidefines abilib abifiles abidirs)
         file(COPY "${incpath}/${fpath}"
           DESTINATION "${CMAKE_BINARY_DIR}/include/${dstdir}"
           )
-        install(FILES "${CMAKE_BINARY_DIR}/include/${fpath}"
-          DESTINATION include/c++/v1/${dstdir}
-          COMPONENT libcxx
-          PERMISSIONS OWNER_READ OWNER_WRITE GROUP_READ WORLD_READ
-          )
+        if (LIBCXX_INSTALL_HEADERS)
+          install(FILES "${CMAKE_BINARY_DIR}/include/${fpath}"
+            DESTINATION include/c++/v1/${dstdir}
+            COMPONENT libcxx
+            PERMISSIONS OWNER_READ OWNER_WRITE GROUP_READ WORLD_READ
+            )
+        endif()
         list(APPEND abilib_headers "${CMAKE_BINARY_DIR}/include/${fpath}")
       endif()
     endforeach()