configure: use the threaded resolver backend by default if possible

configure.ac

@@ -3397,11 +3397,8 @@ else
 fi
 AM_CONDITIONAL(BUILD_LIBHOSTNAME, test x$build_libhostname = xyes)
 
-CURL_CHECK_OPTION_THREADED_RESOLVER
-
-if test "x$want_thres" = xyes && test "x$want_ares" = xyes; then
-  AC_MSG_ERROR(
-[Options --enable-threaded-resolver and --enable-ares are mutually exclusive])
+if test "x$want_ares" != xyes; then
+  CURL_CHECK_OPTION_THREADED_RESOLVER
 fi
 
 dnl ************************************************************

m4/curl-confopts.m4

@@ -5,7 +5,7 @@
 #                            | (__| |_| |  _ <| |___
 #                             \___|\___/|_| \_\_____|
 #
-# Copyright (C) 1998 - 2013, Daniel Stenberg, <daniel@haxx.se>, et al.
+# Copyright (C) 1998 - 2017, Daniel Stenberg, <daniel@haxx.se>, et al.
 #
 # This software is licensed as described in the file COPYING, which
 # you should have received as part of this distribution. The terms
@@ -37,12 +37,12 @@ AC_HELP_STRING([--enable-threaded-resolver],[Enable threaded resolver])
 AC_HELP_STRING([--disable-threaded-resolver],[Disable threaded resolver]),
   OPT_THRES=$enableval)
   case "$OPT_THRES" in
-    yes)
-      dnl --enable-threaded-resolver option used
-      want_thres="yes"
-      ;;
     *)
       dnl configure option not specified
+      want_thres="yes"
+      ;;
+    no)
+      dnl --disable-threaded-resolver option used
       want_thres="no"
       ;;
   esac