Add a flag that allows building standard targets in Emscripten.
Add a flag that allows building standard targets in Emscripten.

Downstream users sometimes need the standard Draco library targets,
even when building in Emscripten. Add the flag ENABLE_JS_GLUE, which
defaults to ON. When turned OFF, the standard Draco library targets
will still be built within an Emscripten build.

Also do some whitespace clean up while working in CMakeLists.txt.

https://github.com/google/draco/issues/407

google/draco,google/draco,louquillio/draco,louquillio/draco,google/draco,louquillio/draco,google/draco,google/draco,google/draco,louquillio/draco,google/draco,louquillio/draco,louquillio/draco