Add CMake for generating javascript protobuf definitions using protoc

[grahamaj]

CMake has a couple of functions to generate protobuf definitions: protobuf_generate_cpp and protobuf_generate_python (See the documentation). Protoc actually supports serveral more languages including javascript. (See the documentation)

protobuf_generate_cpp and protobuf_generate_python both use an undocumented function protobuf_generate. This function almost supports javascript generation, but there are a couple of problems. It doesn't allow you to set additional options for the javascript independent of the output directory (See here and here ) The two most notable options are:

1. --import_stye: allows you to specify the module syntax for the generated code. The default is closure which is incompatible with nodejs
2. binary: this allows you to include parsers for the binary representation. Without this, the generated code is almost useless. Below is a working function that could be included in externpro to facilitate generation of javascript definitions. It is losely based on the cmake protobuf_generate function:

   function(PROTOBUF_GENERATE_JS SRCS)
   if(NOT ARGN)
   message(SEND_ERROR "Error: PROTOBUF_GENERATE_JS() called without any proto files")
   return()
   endif()
   set(generated_srcs)
   foreach(proto_src ${ARGN})
   get_filename_component(abs_file ${proto_src} ABSOLUTE)
   get_filename_component(abs_dir ${abs_file} DIRECTORY)
   get_filename_component(basename ${proto_src} NAME_WE)
   set(generated_src ${abs_dir}${basename}_pb.js)
   list(APPEND generated_srcs ${generated_src})
   add_custom_command(
     OUTPUT ${generated_src}
     COMMAND xpro::protoc
     ARGS --proto_path=${abs_dir} --js_out=import_style=commonjs_strict,binary:${abs_dir} ${abs_file}
     DEPENDS ${abs_file} xpro::protoc
     COMMENT "Running js protobuf compiler on ${proto_src}"
     VERBATIM)
   endforeach()
   set(${SRCS} ${generated_srcs} PARENT_SCOPE)
   endfunction()

[grahamaj]

One more thing, The protoc generated javascript requires a dependency on a library: https://www.npmjs.com/package/google-protobuf