use proper SDL inclusion path

using SDL/*.h, the right headers are automatically picked up
from the system include dir (usually /usr/include), without needing
to add any special CPPFLAGS to search in.
in the very unlikely case that someone has his SDL headers not in
the system or crosscompiler include dir, he has to add a special
include dirs to CPPFLAGS anyway.
gui-sdl: fix compilation on non GP2X
gui-sdl: fix menu.h for inclusion in other TUs
gui-sdl: implement reset
gui-sdl: fix unrepainted mousecursor trash outside the menu window

we simply dont allow it anymore to be moved outside the menu.
gui-sdl: fix fugly green non-transparent mousecursor bg
gui-sdl: fix improper usage of SDL_Quit

actually using it is not even needed, as it's added to atexit()
handlers in main.c.
gui-sdl: make write_text take const char*, and use header decl
gui-sdl: use SDL_DisplayFormat to create surface with copied attrs
gui-sdl: write_text: use beginning and end of too long strings

previously it just wrote outside the screen buffer when encountering
a long filename.

now the first and last 20 bytes are taken and the middle part replaced
with "...". this allows to see the file extension and disk number,
which is usually at the end, as well as the first part of the game
name.
gui-sdl: introduce menu_load_surface and menu_restore_surface

that way the surfaces dont have to be created all over again
and again... and they weren't even freed -> memleaks en masse.
gui-sdl: save and restore amiga screen contents upon entering menu
gui-sdl: major functionality enhancement

- implement loading/saving of savestates (storage menu)
- add possibility to enter fullscreen with ALT-Enter (only from menu)
- implement preferences for floppy speed, sound, RAM, etc
- fix buffer overblows in dirz() with filenames > 64
- only redraw menu when necessary. reduces cpu load from 100% to 2%
  while menu is open.
gui-sdl: set proper keyrepeat while in menu
Merge pull request #37 from rofl0r/sdl-gui-rebased-on-master

make sdl gui usable