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...