Figure out how to use protobufs with meson

[sarum9in]

github.com/meson/mesonbuild does not support protocol buffers well. There is a way to make generator but it is impossible to specify dependencies in a sane way. Also it is not clear how to include headers from generator output.

protoc = find_program('protoc')
protoc_cpp_gen = generator(protoc,
    output    : ['@[email protected]',
                 '@[email protected]'],
    arguments : ['--proto_path=@CURRENT_SOURCE_DIR@/include',
                 '--cpp_out=@BUILD_DIR@', '@INPUT@'])

# To generate
srcs = protoc_cpp_gen.process(
    full_path_to_protos,
    preserve_path_from : join_paths(meson.current_source_dir(), 'include'),
)