net-snmp: fix cross-compile breakage, add libnl pkgconfig option

This patch enhances a previously unapplied patch on jethro, plus adds
some flexibility in terms of required deps and a few cleanups.

Signed-off-by: Stephen Arnold <stephen.arnold42@gmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>