Initial merge of Emscripten port!

With this commit, you can compile SDL2 with Emscripten
( http://emscripten.org/ ), and make your SDL-based C/C++ program
into a web app.

This port was due to the efforts of several people, including: Charlie Birks,
Sathyanarayanan Gunasekaran, Jukka Jyl?nki, Alon Zakai, Edward Rudd,
Bruce Mitchener, and Martin Gerhardy. (Thanks, everyone!)

include/SDL_hints.h

@@ -532,6 +532,20 @@ extern "C" {
  */
 #define SDL_HINT_IME_INTERNAL_EDITING "SDL_IME_INTERNAL_EDITING"
 
+/**
+ *  \brief override the binding element for keyboard inputs for Emscripten builds
+ *
+ * This hint only applies to the emscripten platform
+ *
+ * The variable can be one of
+ *    "#window"      - The javascript window object (this is the default)
+ *    "#document"    - The javascript document object
+ *    "#screen"      - the javascript window.screen object
+ *    "#canvas"      - the WebGL canvas element
+ *    any other string without a leading # sign apples to the element on the page with that ID.
+ */
+#define SDL_HINT_EMSCRIPTEN_KEYBOARD_ELEMENT   "SDL_EMSCRIPTEN_KEYBOARD_ELEMENT"
+
 /**
  *  \brief  An enumeration of hint priorities
  */