Add script to build DCMTK with emscripten support

Building dcmdump.js, dcmxml.js, ... can easily be done using a command
similar to this one:

  cmake -DEMSCRIPTEN_ROOT_PATH:PATH=/path/to/emscripten ../dcmjs

EMSCRIPTEN_ROOT_PATH is the directory containing "emcc", "emc++", ...

It is assumed that the Emsscripten.cmake toolchain file will be located in
${EMSCRIPTEN_ROOT_PATH}/cmake/Platform
Update DCMTK project to include automatic gzip'ing of js files.

$ git shortlog a8507c04..6297448
Jean-Christophe Fillion-Robin (1):
      Add post build target to automatically gzip generated .js
Disable automatic loading of executable and keep the runtime env active.

From the documentation [1]:

noInitialRun: If set to true, main() will not be called. The program will
still call global initializers, set up memory initialization, and so forth.
You can then call main() yourself later.

noExitRuntime: If set to true, the runtime is not shut down after run is
called. Shutting down the runtime calls shutdown callbacks, for example
atexit calls. If you want to be able to continue to use the code after
run finishes, it is safer to set this.

$ git shortlog 629744894..7ef3b940
Jean-Christophe Fillion-Robin (2):
      Force gzip'ing of js file even if it already exists.
      Associate a "pre" javascript file to customize generated js files.

[1] https://github.com/kripken/emscripten/wiki/Interacting-with-code
Add build instruction for cross compiling using CMake