xlib_libXt: fix build issues

The .mk file was hacking the CC_FOR_BUILD value so that
$(STAGING_DIR)/usr/include is added to the include path. This is not
correct since $(STAGING_DIR) contains target stuff, not host
stuff. The correct fix is to add a dependency on host-xproto_xproto,
which will install the needed headers in $(HOST_DIR)/usr/include.

In addition to that, a patch is added to make xlib_libXt build system
behave properly in the cross-compilation case, where the makestrs tool
needs to be built on the host as part of the compilation process. This
was working before because of our quirky TARGET_CONFIGURE_OPTS, but
those are going to be fixed in a future commit.

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