Improve cmake for sdl2

Adds more support to CMake so that building SDL2 via CMakeLists.txt works with minimal changes. See https://github.com/gsathya/SDL-emscripten/issues/3
CMake doesn't need emcc to act any different than it normally does when configure-ing. The toolchain file teaches about the compiler for CMake, and CMake will use that compiler also during configure builds. When debugging SDL2 config, I see that EMMAKEN_JUST_CONFIGURE triggers a bunch of heuristics (in `emcc`, starting with the block `if CONFIGURE_CONFIG or CMAKE_CONFIG:`), and just skipping all those makes CMake happy. With EMMAKEN_JUST_CONFIGURE, emcc would generate native executable files without suffix .js, and CMake would get confused, since it expected .js files to be found.