Build protoc plugins in release mode (#647)
Motivation:
It's easy for users to build the protoc plugins in debug mode (the
default for the Makefile) without realising. They then have to dig
through the build output to find the right binaries. This should be
easier for users.
Modifications:
- Build the plugins in release mode when invoked via `make plugins`
- Copy the plugins to the current directory after building them
Result:
Easier to find the protoc plugins.
George Barnett