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 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...