Detect sccache in RUSTC_WRAPPER and use it for C++ builds?

[mqudsi]

A step below automatically searching for and using sccache (in which case you have to figure out whether or not the intent is to cache the build result, briefly discussed in #252), if RUSTC_WRAPPER is set and uses sccache, would you be open to a PR that automatically used sccache in this case?

[alexcrichton]

Seems plausible to me!