Possible bug compiling Audit 2.6.x against kernel headers older than 3.19

[Adam Duskett]

Audit 2.6.x checks for AUDIT_FEATURE_VERSION to be defined in
include/linux/audit.h (this define was introduced in kernel version
3.13) and then blindly assumes that struct audit_status has
feature_bitmap.

However this looks like a problem as feature_bitmap wasn't introduced
until kernel 3.19, even though the requirements for Audit state that anything >
2.6.30 should work.  (http://people.redhat.com/sgrubb/audit/)

This causes a compile error when attempting to compile audit 2.6.x
against kernel headers younger than 3.19.

libaudit.c:609:33: error: 'struct audit_status' has no member named
'feature_bitmap'
features_bitmap = rep.status->feature_bitmap;