support sccache for the protobuf cmake build

[doy-materialize]

this will try to autodetect if you are using RUSTC_WRAPPER=sccache for your top level build, but will also allow you to set it explicitly.

[benesch]

As a general rule I like to keep this kind of stuff out of the build systems of the crates themselves, and push them into the downstream system that wants to drive the build in a particular way. (For example none of the crates we maintain attempt to do anything special for cross compiling—it's on the user to provide the right compiler toolchain and set the env vars in the right way.) In this case, can you just set the compiler launcher as environment variables when you compile whatever downstream crate you're compiling?

CMAKE_C_COMPILER_LAUNCHER=sccache
CMAKE_CXX_COMPILER_LAUNCHER=sccache

Or is there a particular reason that you need them set for this crate but not other crates?

[doy-materialize]

ah, i didn't realize it worked that way! yeah, environment variables seem fine here.

[benesch]

Ah ok, sounds great!