netsnmp: handle libnl dependency properly

netsnmp: handle libnl dependency properly

libnl3 include path was hardcoded in the configure script,
causing the following gcc warning:

WARNING: unsafe header/library path used in cross-compilation:
'/usr/include/libnl3'

Let's apply an upstream patch that addresses the issue: now
pkg-config is used to detect libnl include path.

Also:
- add optional libnl build dependency;
- enable libnl support only when libnl package is selected.

Signed-off-by: Julien Floret <julien.floret@6wind.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>