linux:/ # make
cc -lX11 -L/usr/X11R6/lib -c -o XWin01.o XWin01.c
cc: -lX11: linker input file unused because linking not done
gcc XWin01.o -o XWin01
XWin01.o(.text+0x22): En la función `main':
XWin01.c: undefined reference to `XOpenDisplay'
XWin01.o(.text+0x53):XWin01.c: undefined reference to `XCreateSimpleWindow'
XWin01.o(.text+0x76):XWin01.c: undefined reference to `XSetStandardProperties'
XWin01.o(.text+0x87):XWin01.c: undefined reference to `XMapWindow'
XWin01.o(.text+0x95):XWin01.c: undefined reference to `XFlush'
collect2: ld returned 1 exit status
make: *** [XWin01] Error 1
¿no deberías incluir alguna librería para manejo de ventanas?. Algo como xwindows.h o algo asi....