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!)

test/testautomation_surface.c

@@ -8,6 +8,7 @@
 #define _CRT_NONSTDC_NO_DEPRECATE
 
 #include <stdio.h>
+#include <unistd.h>
 #include <sys/stat.h>
 
 #include "SDL.h"