package/libnids: disable libnet only if not available

package/libnids: disable libnet only if not available

libnids can be built against an existing libnet lib,
take advantage of that.

Unfortunately, pkg-config is not available for libnet,
so configure --with-libnet is our only option at the
moment.

--with-libnet expects a build directory,
not an installation path like in our context.
We use --with-libnet=yes to skip tests that would fail.
In this situation, 'LIBNET' goes undefined, so we
need to define it ourselves.
This works because we make sure -lnet is installed
prior anything related to libnids.

Signed-off-by: Guillaume W. Bres <guillaume.bressaix@gmail.com>
[yann.morin.1998@free.fr: slightly compact the code]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>