Adds default value "protoc" for protoc path.

This is to avoid having to add machine and OS specific information in the build file. With this change, if no path is specified, it uses "protoc" in the current PATH environment, which should be valid in most cases, but can still be modified if necessary.

It preserves the actual beahviour, except that when nothing is defined, instead of showing an error message, it will try to execute protoc, which seems to be the most sensible thing to do.