CXX should not be hardcoded to g++

[abbec]

When using nix to create a custom derivation on macOS for building protoc-gen-grpc-web I ran into an issue where the protobuf libraries in nixpkgs are compiled with clang and the Makefile inside javascript/net/grpc/web hardcodes CXX to g++.

It builds fine if CXX is set to clang++ so I would propose changing the Makefile line to read

CXX ?= g++
...

to default to g++ but still allow overrides.

[stanley-cheung]

@abbec Thanks for the issue report!