Add a flag for cmake to generate IE11 compatible JS decoder modules

[ggetz]

The current JavaScript decoder module fails when run in IE11.

This allows an IE_COMPATIBLE variable to be set when generating the cmake files in order to build a module that works in IE11 by providing polyfills, at a small hit to module size (1MB), with the solution proposed in emscripten-core/emscripten#6204

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