build.py should not import emscripten python internals

[sbc100]

build.py currently does import tools.shared as emscripten then uses it for a few things.

This kind of use of emscripten internals is almost certainly going to break over time. Lets try to find way to do without it.

[sbc100]

See https://github.com/emscripten-core/emscripten/issues/10939

[kripken]

Probably the best thing would be to rewrite the build script in that case. I think a plain Makefile or CMake would be good ideas.