Fix Makefile compiling same file twice and overriding the correct browser version (ENVIRONMENT=$(ENVIRONMENT)) with Node.js one (ENVIRONMENT=node).
Looks like this was introduced by accident in 33d60658cd.
Fixes#946.
Use a shared base image with fixed Emscripten version, autotools and optimisation flags for all C++ codecs.
Additionally, move build commands for codecs themselves to Makefile - they're already platform-specific, and Make allows for better caching and parallelisation that custom ad-hoc scripts.
This is essentially same as #777 but for C++.
