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