Add support for dynamic libraries in Emscripten

[pixelherodev]

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[floooh]

Thanks, I'll need to test this first on how it affects existing projects (most importantly whether it affects output size in any way).

[pixelherodev]

I'll see about checking code size - I'm requesting the change, it wouldn't be right to take your time on this. For now, I'm more than happy to use my branch of this.

[floooh]

Hmm ok, I'm seeing a few problems on my projects:

• the Closure compiler cannot run on shared modules: assert not options.use_closure_compiler, 'cannot use closure compiler on shared modules'
• the settings.js docs in the emscripten SDK says that with -s LINKABLE=1, no dead code elimination will be performed, this will increase code size in many cases

I guess the proper solution would be to put the shared-module support behind a CMake option which is off by default.

I'll keep this PR open, and when I have time try to take care of it. It will be a little while until I'll find time for this though :)

[pixelherodev]

I see why LINKABLE should be an option, but why would the first one be a problem? It only affects shared modules which don't work without this anyways.

[floooh]

For the first point (about the Closure compiler not working):

I think the proper solution would be to put the shared module settings behind a cmake option, so that they can be switched on/off (e.g. if I want to use the Closure compiler, I would switch the shared module settings off).

My build failed even without building shared modules, which is a bit odd.

Maybe we need (at least) 2 cmake options (build shared modules, and use shared module)...