ウィンドウ出すだけならこんなかんじ。

#include <X11/Xlib.h>
#include <stdio.h>

int main(void)
{
int width = 640;
int height = 480;
int border = 2;
Display *display = XOpenDisplay(NULL);
Window window = XCreateSimpleWindow(
display, RootWindow(display, 0), 0, 0, width, height, border,
BlackPixel(display, 0), WhitePixel(display, 0));

XMapWindow(display, window);
XFlush(display);

getchar();

XDestroyWindow(display, window);
XCloseDisplay(display);

return 0;
}