build: use if_changed on built_in.o

build: use if_changed on built_in.o

In the case where $(obj-y) is empty, we also replace $(c_flags) by
$(XEN_CFLAGS) to avoid generating an .%.d dependency file. This avoid
make trying to include %.h file in the ld command if $(obj-y) isn't
empty anymore on a second run.

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