build: xen/include: use if_changed

build: xen/include: use if_changed

Use "define" for the headers*_chk commands. That allow us to keep
writing "#include" in the Makefile without having to replace that by
"$(pound)include" which would be a bit less obvious about the command
line purpose.

Adding several .PRECIOUS as without them `make` deletes the
intermediate targets. This is an issue because the macro $(if_changed,)
check if the target exist in order to decide whether to recreate the
target.

Removing the call to `mkdir` from the commands. Those aren't needed
anymore because a rune in Rules.mk creates the directory for each
$(targets).

Remove "export PYTHON" as it is already exported.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>