Preprocessor: Use eval() for good

[kripken]

This replaces our custom preprocessor code with eval(). This allows arbitrary conditions in our #ifs, e.g. the __EMSCRIPTEN_HAS_nodefs__ things are now

#if LibraryManager.has('library_workerfs.js')

where that function is a new utility that checks if a library was included.

This patch also switches to things we've wanted to do, like

#if FULL_ES2 || LEGACY_GL_EMULATION

where in the past we had to create a new global for them.

We now error if we do if we ifdef on something nonexistent (the eval call throws), which noticed that we had a bunch of NO_DYNAMIC_EXECUTION code that could be removed (we removed that option a while ago).

[jgravelle-google]

;_; but yeah this is so much better

[jgravelle-google]

Should also yank out the WASM_BACKEND_WITH_RESERVED_FUNCTION_POINTERS definition from jsifier.js, because this should be all that uses it.

[kripken]

Thanks, fixed

[kripken]

I undid the embind changes regarding NO_DYNAMIC_EXECUTION - I realized those were wrong. We will need a different solution there, I think there's a bug in the usage of that flag.

[kripken]

Turns out #7653 will fix the NO_DYNAMIC_EXECUTION stuff which was an unrelated embind bug. So we just need to wait on that.

[kripken]

Actually, this blocks #7612, so let's land it with a small workaround for that embind issue, that we can remove soon.

[wffurr]

Thanks for this - I was considering making #if on a non-existent setting an error as well. Happy to see you have already done so.