Solidity javascript compiler (emscripten target).

I did not change the way boost is discovered for windows. It can be argued, though, whether we want emscripten to also work on windows. I did not do that for now since I do not have a windows machine.

find_package cannot be used with emscripten, because there is no support for the emscripten "architecture" in the usual cmake distributions. Emscripten rather adds a wrapper around cmake to make it work in most cases. find_package is not one of those cases.