Add functions em_link_js_library(), em_link_pre_js() and em_link_post_js() to support linking .js files to executables, while doing dependency tracking on filestamp modifications. For more info, see https://groups.google.com/forum/#!topic/emscripten-discuss/uRbTIB62V7s .
Remove code duplication on CMake link macros.
Merge pull request #1611 from juj/cmake_link_js

Cmake link js