Add emscripten platform to makefile

[JoeOsborn]

This seems to allow dosbox-pure to build on the emscripten platform. The core begins to load but doesn't seem to fully work in a browser; that said, I can't really get any Emscripten core I've compiled myself to work in a browser outside of the official release packages. So it might be working, but I can't figure out how to test it properly.

[JoeOsborn]

Should this be filed against @schellingb ’s repository or this fork?

[LibretroAdmin]

Hi, @schellingb over to you.

[JoeOsborn]

Given libretro/RetroArch#15033 this may work much better! I didn’t realize at the time that the pthread builds weren’t as well supported on the RA side. Is there a way to force RA to build dosbox with pthreads on?

[LibretroAdmin]

Hi, I tested this on our buildbot - it doesn't compile -

https://git.libretro.com/libretro/dosbox-pure/-/jobs/3284326

[JoeOsborn]

Is there a way to tell the buildbot to compile this core with pthread=2 or pthread=4? This core seemingly won’t work without threads because of how it connects libretro api with the dosbox system.

[JoeOsborn]

That said, I was seeing weird phenomena like DOS out of memory errors for Commander Keen 1 and freezes for Keen 4 and Wolf3D, and I’m not familiar enough with the dosbox code to make attempts at diagnosing those. So let’s disable dosbox-pure for emscripten even though it does compile, unless someone can own the fixes.