build: generate "include/xen/compile.h" with if_changed

build: generate "include/xen/compile.h" with if_changed

This will avoid regenerating "compile.h" if the content hasn't changed.

As it's currently the case, the file isn't regenerated during `sudo
make install` if it exist and does belong to a different user, thus we
can remove the target "delete-unfresh-files". Target "$(TARGET)" still
need a phony dependency, so add "FORCE".

Use "$(dot-target).tmp" as temporary file as this is already cover by
".*.tmp" partern in ".gitconfig".

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