Add SYSTEM_UDNS CMake option. Remove spurious EXPAT_LIBRARIES linking when using bundled expat.

CMakeModules/FindUdns.cmake

@@ -0,0 +1,42 @@
+# - Try to find UDNS library
+# Once done this will define
+#
+#  UDNS_FOUND - system has UDNS
+#  UDNS_INCLUDE_DIRS - the UDNS include directory
+#  UDNS_LIBRARIES - Link these to use UDNS
+#  UDNS_DEFINITIONS - Compiler switches required for using UDNS
+#
+#  Copyright (c) 2016 Maciej Mrozowski <reavertm@gmail.com>
+#
+# Distributed under the Boost Software License, Version 1.0.
+# (See accompanying file LICENSE_1_0.txt or copy at
+# http://www.boost.org/LICENSE_1_0.txt)
+
+
+if (UDN_LIBRARIES AND UDN_INCLUDE_DIRS)
+  # in cache already
+  set(UDNS_FOUND TRUE)
+else ()
+  set(UDNS_DEFINITIONS "")
+
+  find_path(UDNS_INCLUDE_DIRS NAMES udns.h)
+  find_library(UDNS_LIBRARIES NAMES udns)
+
+  if (UDNS_INCLUDE_DIRS AND UDNS_LIBRARIES)
+    set(UDNS_FOUND TRUE)
+  endif ()
+
+  if (UDNS_FOUND)
+    if (NOT Udns_FIND_QUIETLY)
+      message(STATUS "Found UDNS: ${UDNS_LIBRARIES}")
+    endif ()
+  else ()
+    if (Udns_FIND_REQUIRED)
+      message(FATAL_ERROR "Could not find UDNS")
+    endif ()
+  endif ()
+
+  # show the UDNS_INCLUDE_DIRS and UDNS_LIBRARIES variables only in the advanced view
+  mark_as_advanced(UDNS_INCLUDE_DIRS UDNS_LIBRARIES)
+
+endif ()