net-snmp: add knob whether nlist.h are checked

Previously, it still was checked when there was no nlish.h in sysroots directory.
Add knob to decide whether nlist.h are checked or not.

Fixed by using PACKAGECONFIG to check elf, with default disabled set.

Signed-off-by: Chong Lu <Chong.Lu@windriver.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>