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/configure

@@ -2980,6 +2980,11 @@ fi
         MATHLIB=""
         SYS_GL_LIBS="-lGLES_CM"
         ;;
+    *-*-emscripten* )
+                EXE=".bc"
+        MATHLIB=""
+        SYS_GL_LIBS=""
+        ;;
     *)
                 ISUNIX="true"
         EXE=""