libsdl: config tweaks, remove sdlmain lib

This patch contains changes to the libSDL port that were needed to build
the Hatari emulator using cmake.

- Since SDL programs cannot use the posix library (because sdl's init
  function needs a way to get hold of the Genode env), the sdl_main
  library must be used instead of the posix library. The sdl_main
  code implements the Libc::Component::construct function for the
  final binary. However, 3rd-party build systems are not aware of this
  Genode-specific peculiarity and thereby create binaries that leave
  libsdl uninitialized. This patch adds the sdl_main to libsdl by
  default and thereby eliminates the need for specifying the extra
  sdl_main library. Customizing the Libc::Component::construct is
  still possible by overriding the library-provided implementation
  in the main program.

- A few missing config definitions are added to compile Hatari
  successfully.

- Avoid reliance on the include/SDL convention in SDL_config.h

- Cmake relies on the upper-case naming of the library with no way
  to tweak this convention. So we install an symlink named `SDL.lib.so`
  when installing the library into the <build-dir>/bin/ directory.

Fixes #161

lib/symbols/sdl

@@ -1,3 +1,4 @@
+_ZN4Libc9Component9constructERNS_3EnvE T
 SDL_AddTimer T
 SDL_AllocBlitMap T
 SDL_AllocFormat T