Use the array_size() helper macro.  Replaces several instances of local
macros with the same definition.

Reviewed-by: Scott Feldman <sfeldma@cumulusnetworks.com>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>

The correct method to link to NetSNMP is to use net-snmp-config (which
is like pkg-config). Explicit link to libcrypto is also dropped
(NetSNMP libs are linked to libcrypto, no need to link Quagga to
it). Moreover, @SNMP_INCLUDES@ is dropped because useless. Due to a
bug in configure.ac, it was properly populated.

Some .h files in lib/ are autogenerated. The search path should
include the build directory and the source directory. They usually
match but sometimes, they may be different. For example:

 $ mkdir build
 $ cd build
 $ ../configure
 $ make

In upstream babel, we include different kernel_foo files depending on the
platform we compile for.  In Quagga, this is not needed, and it avoids
tricky autotools issues.  (All autotools issues are tricky.)

[David: without this commit, build breaks.]

From: Juliusz Chroboczek <jch@pps.jussieu.fr>
Signed-off-by: David Lamparter <equinox@diac24.net>

babeld wouldn't build in a dist tarball without this.

From: Denis Ovsienko <infrastation@yandex.ru>
Signed-off-by: Juliusz Chroboczek <jch@pps.jussieu.fr>
[fixed up git message]
Signed-off-by: David Lamparter <equinox@diac24.net>

This is the functionality described in Appendix C of RFC 6126.  Its
main purpose is to avoid keeping a full source table, which makes it
possible to implement a subset of Babel in just a few hundred lines of
code.  However, in Quagga the code for maintaining the source table is
already there, and a parasitic implementation can be simulated using
filtering -- so it makes little sense to keep the functionality.

This commit makes the following lines visible in running-config text,
when respective intervals are configured to non-default values:

* babel hello-interval
* babel update-interval
* babel resend-delay