SRV lookup library

This uses the ADNS library, if available. Otherwise, the host program.

I anticipate that cabal users won't easily get hsdns installed, since
it's a Haskell binding to a C library. And using host is just fine, as
long as the system has it.

Build/Configure.hs

@@ -27,6 +27,7 @@ tests =
 	, TestCase "bup" $ testCmd "bup" "bup --version >/dev/null"
 	, TestCase "gpg" $ testCmd "gpg" "gpg --version >/dev/null"
 	, TestCase "lsof" $ testCmd "lsof" "lsof -v >/dev/null 2>&1"
+	, TestCase "host" $ testCmd "host" "host localhost >/dev/null 2>&1"
 	, TestCase "ssh connection caching" getSshConnectionCaching
 	] ++ shaTestCases
 	[ (1, "da39a3ee5e6b4b0d3255bfef95601890afd80709")

git-annex.cabal

@@ -46,6 +46,9 @@ Flag Pairing
 Flag XMPP
   Description: Enable notifications using XMPP
 
+Flag Adns
+  Description: Enable the ADNS library for DNS lookup
+
 Executable git-annex
   Main-Is: git-annex.hs
   Build-Depends: MissingH, hslogger, directory, filepath,
@@ -98,6 +101,10 @@ Executable git-annex
     Build-Depends: network-protocol-xmpp
     CPP-Options: -DWITH_XMPP
 
+  if flag(XMPP) && flag(Assistant) && flag(Adns)
+    Build-Depends: hsdns
+    CPP-Options: -DWITH_ADNS
+
 Test-Suite test
   Type: exitcode-stdio-1.0
   Main-Is: test.hs