Generate scno.h without invoking a generated executable helper

Generate scno.h without invoking a generated executable helper

Generate scno.h using only those tools that are available for cross
build.  This allows to use scno.h during the build of strace itself.

* scno.am: New file.
* scno.head: New file.
* Makefile.am (EXTRA_DIST): Add it.
* tests/generate_scno.c: Remove.
* tests/.gitignore: Remove generate_scno.
* tests/Makefile.am (check_PROGRAMS): Likewise.
(CLEANFILES): Remove scno.h.
(scno.h, $(scno_dependants)): Remove rules.
(scno_dependants): Remove.
($(objects)): New rule.
Include ../scno.am.