syslog-ng: add optional support for libnet build

syslog-ng: add optional support for libnet build

If libnet is included pass --enable-spoof-source to configure otherwise
pass --disable-spoof-source. This stops syslog-ng's configure from
enabling address spoofing based on the build host having libnet
installed.

Fixes:

  http://autobuild.buildroot.net/results/71c733e4a7275f3a1b79cd19065d94e77af0309b
  http://autobuild.buildroot.net/results/d16ba0b9ef75d5a676a26db788be31556b69772a
  http://autobuild.buildroot.net/results/9659d1db1a128b269459fd9b8be797a9efbd35af

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>