tools: add compile_commands.json gyp generator

Add a compile_commands.json generator for use with clang-based tooling.

Pass the (undocumented) -C switch to configure to make it generate the
files in out/Debug and out/Release.

PR-URL: https://github.com/nodejs/node/pull/7986


Reviewed-By: James M Snell <jasnell@gmail.com>