Update emscripten cmake

[rdroe]

Closes OGStudio/openscenegraph-cross-platform-guide/issues/8.
After this, Emscripten seems to compile.

As an added note, some warnings remain and end of build process. I think they were already there.

[ 99%] Linking CXX executable ../../bin/osgstaticviewer.js
[ 99%] Built target osgdb_p3d
[100%] Linking CXX executable sample-ems.html
warning: undefined symbol: pthread_attr_getstacksize
warning: undefined symbol: pthread_testcancel
warning: undefined symbol: graphicswindow_X11
warning: undefined symbol: pthread_attr_getstacksize
warning: undefined symbol: pthread_testcancel
[100%] Built target sample-ems
[100%] Built target example_osgstaticviewer