Update DCMTK version to 2014-11-03-6a7c73f
Update build system to work with Emscripten >= 1.21.4

* Recent version of Emscripten SDK provide CMake file into Modules subdir.
* .js suffix automatically appended.

See kripken/emscripten@6038404
Add check for Emscripten version
Merge branch 'update-emscripten-and-dcmtk'

* update-emscripten-and-dcmtk:
  Add check for Emscripten version
  Update build system to work with Emscripten >= 1.21.4
  Update DCMTK version to 2014-11-03-6a7c73f