Avoid generating *.map files or running headers.chk when all we need
is the libxen*.a.
Also, allow force make to check again if libxen*.a needs rebuilt by
adding a '.PHONY' prerequisite.
Also, remove DESTDIR= as we don't do installation in this target, so
the value of DESTDIR doesn't matter.
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>