Add support for compiler caches

- Add support for compiler wrappers in SCons (Disabled by default).
- In tools/test.py: enable ccache if it is available.
- Set the environment variable CCACHE_CPP2 to 'yes' to fix warnings on
ubuntu 16.04.

Change-Id: If8d1b094143a5c1a9c58fd7c204655f8889d34fd