core: Make cross building in mkheader more explicit.

* src/mkheader.c (main): Add option --cross.
(write_special): Don't use native in cross mode.
* src/Makefile.am (mkheader_opts): New.
(gpg-error.h): Add MKHEADER_OPTS.

The old behaviour was that an existing lock-obj-oub.native.h was
preferred over one from syscfg even in cross mode. The Makefile tried
to get this right by deleting such a file first but when calling
mkheader manually for testing this may have not been done. We now
allow (and also use by the Makefile) an option --cross to make this
explicit.

• Signed-off-by: Werner Koch <wk@gnupg.org>