#8 polymorphic variant for C flags

[Olivier Andrieu]

Sdl.init_flag and Sdlvideo.video_flag are now
polymorhic variants.

The C code is taken from LablGTK. The varcc utility of
LablGTK generates a .h and .c file containing a lookup
table for the conversion C flags <-> ML variant.

sdlinit_flag.c sdlinit_flag.h sdlvideo_flag.c
sdlvideo_flag.h are new files with theses (precomputed)
lookup tables. The code that does the actual lookup is
in common.c and common.h.

If you want to modify the variant types, you'll have to
re-run varcc (not included in the patch).

[Olivier Andrieu]