build: Use gen-lock-obj.sh script when possible.

* configure.ac (HAVE_GENERATED_LOCK_OBJ_H): New.
* src/Makefile.am: Support HAVE_GENERATED_LOCK_OBJ_H.

For cross build, when possible (currently for GNU/Linux),
generate lock-obj-pub.native.h by the script.

• GnuPG-bug-id: T4774
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>