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

include/SDL/SDL_config.h

@@ -31,7 +31,7 @@
 #ifndef _SDL_config_h
 #define _SDL_config_h
 
-#include <SDL/SDL_platform.h>
+#include <SDL_platform.h>
 #include "SDL_config_genode.h"
 
 #endif /* _SDL_config_h */

include/SDL/SDL_config_genode.h

@@ -61,6 +61,8 @@
 
 #define SDL_VIDEO_OPENGL 1
 
+#define SDL_HAS_64BIT_TYPE 1
+
 /* #define HAVE_MREMAP 0 */
 #define HAVE_MALLOC
 #define HAVE_CALLOC
@@ -77,7 +79,9 @@
 #define HAVE_SNPRINTF
 #define HAVE_VSNPRINTF
 
-#define HAVE_STDIO_H 1
+#define HAVE_STDIO_H    1
+#define HAVE_INTTYPES_H 1
+#define HAVE_MATH_H     1
 
 /* #define SDL_malloc	malloc */
 /* #define SDL_calloc	calloc */