xorg: add a new cross-compile fix for libXt 1.0.6

The cross-compile fix included in mainline of libXt between 1.0.5 and
1.0.6 is not sufficient. It allows to pass a different compiler for
the host tools, but still uses the TARGET_CFLAGS/TARGET_LDFLAGS to
compile the host tools, which obviously isn't correct.

The new patch allows to use the CFLAGS_FOR_BUILD/LDFLAGS_FOR_BUILD
variables as the CFLAGS/LDFLAGS to build the host tools.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>