Improve caching.
Improve caching.

* Take into account what compiler what we're when generating the sha.
* Use $CC for C, and $CXX for C++.
* Use $CFLAGS for C, and $CXXFLAGS for C++.
* Use $CPPFLAGS for both.

ryanmjacobs/c,ryanmjacobs/c,ryanmjacobs/c,ryanmjacobs/c