C/C++ check with clang-tidy, auto-find build dir for compilation db.

clang-tidy reads in compile_commands.json form nearest parent dir.

In case with CMake you can set(CMAKE_EXPORT_COMPILE_COMMANDS ON) to generate this file automatically. The result however is placed in the build dir, not root of project. Syntastic_c_build_dir_set() will attempt to find build dir and set it when found. Current file is used for base path, not working directory.

Project template/c will be updated soon, refer to template/c#17 (closed).