Ability to manually set DNS_BLOCK_ASSERTIONS=1

Ability to add
`-DNS_BLOCK_ASSERTIONS=1` to `OTHER_CFLAGS` and `OTHER_CPLUSPLUSFLAGS` with
  `set_dns_block_assertions_flag!` to your Podfile.
Possibly closes [#802](https://github.com/CocoaPods/CocoaPods/issues/802)