Add support for ccache by setting CCACHE in environment.

If you install ccache and set CCACHE=ccache in your environment then you'll get ccache'd builds.

For stmhal:
Without ccache, on my laptop, a clean build takes 26 seconds.
With ccache and an empty cache, a clean build takes 30 seconds
With ccache and a hot cache, a clean build takes 3 seconds

For unix, the corresponding times are: 11.0, 12.5, and 1.1