Add a CMakelists.txt to compile on Linux and Mac

Windows support is still WIP

Change-Id: I5fd5bcda4fad754c0ce73698eddbe73e538f56df
Reviewed-on: https://swiftshader-review.googlesource.com/4370
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>