We've been running each Make command in a single thread, resulting in fairly slow builds for C++ codecs.
This change instead runs all `make` invocations with `-j` defaulting to number of cores (retrieved via `nproc`).
On my machine Docker uses a VM configured to 4 cores out of 8 available. This change brings total build time for C++ codecs down from 10m28s to 7m5s (~3.5 minutes difference).
Note (1): I've converted imagequant builds to use built-in `make` as well to leverage this parallelisation and future-proof build script.
Note (2): we don't need to do the same for Rust, since Cargo parallelises builds by default.
Looks like we've been stuck on Emscripten fastcomp backend, misssing out on all new optimisations between LLVM 6 and LLVM 11 and going via slow asm.js pipeline when building.
This changes Docker images for Emscripten projects to point to emscripten-upstream instead and commits the updated artifacts.
