Commit 5f9b1e35 remove the dependency between OpenGL and X11.
However the milkymist-tmu2 device do require X11.
When using SDL, the configure script sets need_x11=yes, so the X11
flags are populated to the makefiles.
When building without SDL, X11 is not pulled and populated, leading
to a link failure:

    LINK    lm32-softmmu/qemu-system-lm32
  hw/lm32/milkymist.o: In function `milkymist_tmu2_create':
  hw/lm32/milkymist-hw.h:114: undefined reference to `XOpenDisplay'
  hw/lm32/milkymist-hw.h:140: undefined reference to `XFree'
  hw/lm32/milkymist-hw.h:141: undefined reference to `XCloseDisplay'
  hw/lm32/milkymist-hw.h:130: undefined reference to `XCloseDisplay'
  ../hw/display/milkymist-tmu2.o: In function `tmu2_glx_init':
  hw/display/milkymist-tmu2.c:112: undefined reference to `XOpenDisplay'
  hw/display/milkymist-tmu2.c:123: undefined reference to `XFree'
  collect2: error: ld returned 1 exit status
  gmake[1]: *** [Makefile:199: qemu-system-lm32] Error 1

Enforce the X11 dependency when the LM32 target is built.
This will allow us to build QEMU without SDL.
Signed-off-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20190130120005.23123-3-philmd@redhat.com
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>