add --with-dnssec for compiling in optional DNSSEC support in dnsmasq

DNSSEC support is in dnsmasq since [2.69test5](http://lists.thekelleys.org.uk/pipermail/dnsmasq-discuss/2014q1/008086.html)
Use it by adding conf options like:
```
dnssec
dnssec-check-unsigned
trust-anchor=.,19036,8,2,49AAC11D7B6F6446702E54A1607371607A1A41855200FD2CE1CDDE32F24E8FB5
```
Trust anchor comes from [IANA](https://data.iana.org/root-anchors/root-anchors.xml)
Your upstream resolvers must be DNSSEC compatible for this to work. [Google Public DNS](https://developers.google.com/speed/public-dns/faq#dnssec) and [dns.watch](https://dns.watch/index) both support DNSSEC. OpenDNS does not at this time.