Fix detection of the AsynchDNS feature which not just depends on
pthreads support, but also on whether USE_POSIX_THREADS is set or not.
Caught by test 1014.

This patch adds a new ENABLE_THREADED_RESOLVER option (corresponding to
--enable-threaded-resolver of autotools) which also needs a check for
HAVE_PTHREAD_H.

For symmetry with autotools, CURL_USE_ARES is renamed to ENABLE_ARES
(--enable-ares). Checks that test for the availability actually use
USE_ARES instead as that is the result of whether a-res is available or
not (in practice this does not matter as CARES is marked as required
package, but nevertheless it is better to write the intent).

Signed-off-by: Peter Wu <peter_at_lekensteyn.nl>

---
 CMakeLists.txt               | 27 +++++++++++++++++++++------
 lib/CMakeLists.txt           |  2 +-
 lib/curl_config.h.cmake      |  6 ++++++
 tests/libtest/CMakeLists.txt |  2 +-
 tests/server/CMakeLists.txt  |  2 +-
 5 files changed, 30 insertions(+), 9 deletions(-)
diff --git a/CMakeLists.txt b/CMakeLists.txt
index a325d1c..57575f4 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -74,12 +74,17 @@ include_directories( ${CURL_SOURCE_DIR}/include )
 option(BUILD_CURL_EXE "Set to ON to build cURL executable." ON)
 option(BUILD_CURL_TESTS "Set to ON to build cURL tests." ON)
 option(CURL_STATICLIB "Set to ON to build libcurl with static linking." OFF)
-option(CURL_USE_ARES "Set to ON to enable c-ares support" OFF)
+option(ENABLE_ARES "Set to ON to enable c-ares support" OFF)
+option(ENABLE_THREADED_RESOLVER "Set to ON to enable POSIX threaded DNS lookup" OFF)
 # initialize CURL_LIBS
 set(CURL_LIBS "")
 
-if(CURL_USE_ARES)
-  set(USE_ARES ${CURL_USE_ARES})
+if(ENABLE_THREADED_RESOLVER AND ENABLE_ARES)
+  message(FATAL_ERROR "Options ENABLE_THREADED_RESOLVER and ENABLE_ARES are mutually exclusive")
+endif()
+
+if(ENABLE_ARES)
+  set(USE_ARES 1)
   find_package(CARES REQUIRED)
   list(APPEND CURL_LIBS ${CARES_LIBRARY} )
   set(CURL_LIBS ${CURL_LIBS} ${CARES_LIBRARY})
@@ -236,6 +241,18 @@ if(WIN32)
   include(${CMAKE_CURRENT_SOURCE_DIR}/CMake/Platforms/WindowsCache.cmake)
 endif(WIN32)
 
+if(ENABLE_THREADED_RESOLVER)
+  check_include_file_concat("pthread.h" HAVE_PTHREAD_H)
+  if(HAVE_PTHREAD_H)
+    set(CMAKE_THREAD_PREFER_PTHREAD 1)
+    find_package(Threads)
+    if(CMAKE_USE_PTHREADS_INIT)
+      set(CURL_LIBS ${CURL_LIBS} ${CMAKE_THREAD_LIBS_INIT})
+      set(USE_THREADS_POSIX 1)
+    endif()
+  endif()
+endif()
+
 # Check for all needed libraries
 check_library_exists_concat("dl"     dlopen       HAVE_LIBDL)
 check_library_exists_concat("socket" connect      HAVE_LIBSOCKET)
@@ -1003,9 +1020,7 @@ _add_if("SSL"           SSL_ENABLED)
 _add_if("IPv6"          ENABLE_IPV6)
 _add_if("unix-sockets"  USE_UNIX_SOCKETS)
 _add_if("libz"          HAVE_LIBZ)
-find_package(Threads)
-# AsynchDNS depends or USE_ARES or pthreads (mutually exclusive)
-_add_if("AsynchDNS"     USE_ARES OR CMAKE_USE_PTHREADS_INIT)
+_add_if("AsynchDNS"     USE_ARES OR USE_THREADS_POSIX)
 _add_if("IDN"           HAVE_LIBIDN)
 # TODO SSP1 (WinSSL) check is missing
 _add_if("SSPI"          USE_WINDOWS_SSPI)
diff --git a/lib/CMakeLists.txt b/lib/CMakeLists.txt
index 78fde4d..e7886ac 100644
--- a/lib/CMakeLists.txt
+++ b/lib/CMakeLists.txt
@@ -57,7 +57,7 @@ include_directories(${CMAKE_CURRENT_SOURCE_DIR}/../include)
 include_directories(${CMAKE_CURRENT_BINARY_DIR}/..)
 include_directories(${CMAKE_CURRENT_SOURCE_DIR})
 include_directories(${CMAKE_CURRENT_BINARY_DIR})
-if(CURL_USE_ARES)
+if(USE_ARES)
   include_directories(${CARES_INCLUDE_DIR})
 endif()
 
diff --git a/lib/curl_config.h.cmake b/lib/curl_config.h.cmake
index 32bae39..44014f5 100644
--- a/lib/curl_config.h.cmake
+++ b/lib/curl_config.h.cmake
@@ -467,6 +467,9 @@
 /* Define to 1 if you have a working POSIX-style strerror_r function. */
 #cmakedefine HAVE_POSIX_STRERROR_R 1
 
+/* Define to 1 if you have the <pthread.h> header file */
+#cmakedefine HAVE_PTHREAD_H 1
+
 /* Define to 1 if you have the <pwd.h> header file. */
 #cmakedefine HAVE_PWD_H 1
 
@@ -879,6 +882,9 @@
 /* Define if you want to enable c-ares support */
 #cmakedefine USE_ARES 1
 
+/* Define if you want to enable POSIX threaded DNS lookup */
+#cmakedefine USE_THREADS_POSIX 1
+
 /* Define to disable non-blocking sockets. */
 #cmakedefine USE_BLOCKING_SOCKETS 1
 
diff --git a/tests/libtest/CMakeLists.txt b/tests/libtest/CMakeLists.txt
index ef73e55..cc9d7e1 100644
--- a/tests/libtest/CMakeLists.txt
+++ b/tests/libtest/CMakeLists.txt
@@ -9,7 +9,7 @@ function(SETUP_TEST TEST_NAME)          # ARGN are the files in the test
     ${CURL_BINARY_DIR}/lib          # To be able to reach "curl_config.h"
     ${CURL_BINARY_DIR}/include      # To be able to reach "curl/curlbuild.h"
     )
-  if(CURL_USE_ARES)
+  if(USE_ARES)
     include_directories(${CARES_INCLUDE_DIR})
   endif()
 
diff --git a/tests/server/CMakeLists.txt b/tests/server/CMakeLists.txt
index 031d817..00f5242 100644
--- a/tests/server/CMakeLists.txt
+++ b/tests/server/CMakeLists.txt
@@ -9,7 +9,7 @@ function(SETUP_EXECUTABLE TEST_NAME)    # ARGN are the files in the test
     ${CURL_BINARY_DIR}/lib      # To be able to reach "curl_config.h"
     ${CURL_BINARY_DIR}/include  # To be able to reach "curl/curlbuild.h"
     )
-  if(CURL_USE_ARES)
+  if(USE_ARES)
     include_directories(${CARES_INCLUDE_DIR})
   endif()
 
-- 
2.1.2
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