Improve compatibility with CMake 3.0 and later

CMake 3.0 changed the identifier for Apple-supplied Clang to
AppleClang.

CMake 3.1 changed the behavior of variable expansion in quoted
strings and complains with warning CMP0054 twice without these
changes.

BUG=crbug.com/451610

Change-Id: I0f1514ec302cf5f1b5cfc2c5a1c71c9e20d5e855
Reviewed-on: https://boringssl-review.googlesource.com/3011
Reviewed-by: Adam Langley <agl@google.com>