QEMU version 2.11 will remove the pixman submodule. This means users are
always required to supply pixman from the system if building softmmu
support in QEMU.
Without specifying a pixman configure option QEMU will default to using
the system pixman if it is avaliable. In which case let's remove the
config option as it is no longer supported in recent commits and is not
required in older builds as the configure system defaults are already
using system pixman.
Signed-off-by: Alistair Francis <alistair.francis@xilinx.com> Signed-off-by: Ross Burton <ross.burton@intel.com>
