Linking with SDL,

I’m trying to compile a small app which uses SDL,
but I’m having trouble, as I cannot fine the X library which has all
these things :

/usr/lib/libSDL.a(SDL_x11gamma.lo): In function X11_SetGammaNoLock': SDL_x11gamma.lo(.text+0x115): undefined reference toXF40VidModeSetGamma’
/usr/lib/libSDL.a(SDL_x11gamma.lo): In function X11_GetGammaNoLock': SDL_x11gamma.lo(.text+0x223): undefined reference toXF40VidModeGetGamma’
/usr/lib/libSDL.a(SDL_x11modes.lo): In function

does anyone know the one ?