Few notes:
- Lots of deprecated SIMD intrinsic warnings & errors in JPEG-XL -> Highway; had to suppress erorrs to make project build.
- Moved couple of common link flags to cpp.Dockerfile (note: can't move `EXPORT_ES6` otherwise `configure` will fail).
- MODULARIZE=1 is no longer necessary and implied by EXPORT_ES6.
- EXPORT_NAME=... is no longer necessary in EXPORT_ES6.
- Changed visdif to also use EXPORT_ES6 and ENVIRONMENT=node instead of generic JS.
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++.
