Ok for real the first commit

external/SDL2/docs/README-emscripten.md

@@ -0,0 +1,35 @@
+Emscripten
+================================================================================
+
+Build:
+
+    $ mkdir build
+    $ cd build
+    $ emconfigure ../configure --host=asmjs-unknown-emscripten --disable-assembly --disable-threads --disable-cpuinfo CFLAGS="-O2"
+    $ emmake make
+
+Or with cmake:
+
+    $ mkdir build
+    $ cd build
+    $ emcmake cmake ..
+    $ emmake 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
+
+Some other SDL2 libraries can be easily built (assuming SDL2 is installed somewhere):
+
+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...