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!)

docs/README-emscripten.txt

@@ -0,0 +1,26 @@
+Build:
+$ emconfigure ./configure --host=asmjs-unknown-emscripten --disable-assembly --disable-threads --enable-cpuinfo=false CFLAGS="-O2"
+$ emmake make
+
+Or with cmake:
+$ emconfigure cmake ..
+$ make
+
+To build one of the tests -
+$ cd test/
+$ emcc -O2 --js-opts 0 -g4 testdraw2.c -I../include ../build/.libs/libSDL2.a ../build/libSDL2_test.a -o a.html
+
+Uses GLES2 renderer or software
+
+tests: https://dl.dropboxusercontent.com/u/17360362/SDL2-em/index.html
+
+Some other SDL2 libraries can be easily built (assuming SDL2 is installed somwhere):
+
+SDL_mixer (http://www.libsdl.org/projects/SDL_mixer/)
+    $ EMCONFIGURE_JS=1 emconfigure ../configure
+    build as usual...
+
+SDL_gfx (http://cms.ferzkopp.net/index.php/software/13-sdl-gfx):
+    $ EMCONFIGURE_JS=1 emconfigure ../configure --disable-mmx
+    build as usual...
+