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!)

build-scripts/config.sub

@@ -1534,6 +1534,8 @@ case $os in
 	-pnacl*)
 		os=-pnacl
 		;;
+	-emscripten*)
+		;;
 	-none)
 		;;
 	*)