Add build_with for custom cc::Build setups

[elrnv]

Added a build_with member function to Config to allow for feeding in custom cc::Build configurations (both C and C++). This makes it particularly convenient, for instance to integrate cmake builds with the cxx-build crate, which produces cc::Build configs.

For instance, we may build C++ code with cxx with cmake using:

let build = cxx_build::bridge("src/lib.rs");
cmake::Config::new(".")
    .no_build_target(true)
    .build_with(build.clone(), build.clone());

This is useful when integrating cxx with existing projects that already use CMake.
I tested this particular workflow in the hdkrs crate, which builds a cxx layer between the Houdini C++ API (which uses CMake) and Rust.

[alexcrichton]

Could these perhaps be dedicated configuration methods rather than a separate build method? Otherwise seems reasonable to me!

[elrnv]

Good point! I updated the PR with two additional configuration options in Config that let the user set an explicit cc::Build starting point for C and C++ (separately).

[alexcrichton]

Looks good to me, thanks!