Ccache support for Emscripten

[Jukka Jylänki]

Hi all,

there are now pull requests available to add support to ccache (
https://ccache.dev/ ) for Emscripten. Ccache speeds up recompilation
of identical code files.

If you are interested in giving it a try ahead of landing, see more
information at
- Emsdk PR: https://github.com/emscripten-core/emsdk/pull/711
- Emscripten PR: https://github.com/juj/ccache/tree/emscripten
- CCache Emscripten fork: https://github.com/juj/ccache/tree/emscripten

As a test case, recompiling Gabriel Cuvillier's Doom 3 Wasm port at
https://github.com/gabrielcuvillier/d3wasm to wasm, which consists of
267 source files:

- cold compile without ccache support enabled: 13.84s
- first time cold compile: 18.65s
- second time hot compile (full rebuild of all files in cache): 2.66s

So rebuilds of identical files speed up quite well, whereas cold
builds do get a bit slower, a tradeoff.

I would be curious to hear user impressions if people run into any bugs.

Cheers,
Jukka