Initial support for compiling a web application using Emscripten.

This plays files in the browser using an audio tag, obtains
samples via the HTML5 audio API, and then passes them to
Synaesthesia. The user supplies files via drag and drop.

Configure via "emconfigure ./configure". Emscripten is detected
in configure.ac. Some kludges are used because Autoconf doesn't
detect Emscripten.