configure: Fix build with XFree

The build is broken on ppc64-linux, possibly only with new binutils:

ld: hw/lm32/../milkymist-tmu2.o: undefined reference to symbol 'XFree'
ld: note: 'XFree' is defined in DSO /lib64/libX11.so.6 so try \
  adding it to the linker command line

So let's follow the linker's advice.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>