Make it easier to build libminijail on Chromium Linux.

- Move libsyscalls.gen.c generation code out of the Makefile and into a
  script.
- Add SECURE_ALL_* defines for systems that do not linux/securebits.h.

BUG=chromium-os:35482
TEST=FEATURES=test emerge chromeos-minijail

Change-Id: I922c579f1fcf09db2379659dbde737f246200e51
Reviewed-on: https://gerrit.chromium.org/gerrit/35928
Reviewed-by: Jorge Lucangeli Obes <[email protected]>
Reviewed-by: Mike Frysinger <[email protected]>
Commit-Ready: Lei Zhang <[email protected]>
Tested-by: Lei Zhang <[email protected]>
3 files changed