Enable PNG support in DCMTK

Also generalize who Emscripten compiler and flags are passed to the
different projects.
Do not associated any langage with top-level project
Merge branch 'add-png-support'

* add-png-support:
  Do not associated any langage with top-level project
  Enable PNG support in DCMTK
Add dcmjs.js - wrapper around a configured set of dcmtk applications

Instead of generating on .js file for each application, a single dcmjs.js
script is now generated. It is basically a wrapper allowing to
execute any dcmtk application.
Merge branch 'generate-dcmjs-wrapper'

* generate-dcmjs-wrapper:
  Add dcmjs.js - wrapper around a configured set of dcmtk applications