rdkafka-sys: don't build C++ libs

[jcaesar]

• My motivation for this is to make cross-compiling easier. This gets rid of the need of a (working) C++ setup for cross compiling
• A small speedup is a welcome side-effect
• I have added this for both mklove and cmake builds, but I do feel like the CMake variant is a bit hacky, using the lib without installing it. If you think this is too brittle, I'd be happy to get only the mklove part accepted or hide it behind an env var.

[rukai]

This PR works for me and reduces the size of target/build/rdkafka-sys-* from 45MB to 23MB which is quite nice.
Would really like to see this land.