FindThreads: move checking of the -pthread compiler flag into a macro

This allows a following commit to introduce a switch to prefer that check over
searching for the explicit library names without breaking backward
compatibility.

Modules/FindThreads.cmake

@@ -22,6 +22,7 @@
 
 #=============================================================================
 # Copyright 2002-2009 Kitware, Inc.
+# Copyright 2011-2014 Rolf Eike Beer <[email protected]>
 #
 # Distributed under the OSI-approved BSD License (the "License");
 # see accompanying file Copyright.txt for details.
@@ -48,7 +49,7 @@ endif()
 # Internal helper macro.
 # Do NOT even think about using it outside of this file!
 macro(_check_threads_lib LIBNAME FUNCNAME VARNAME)
-  if(NOT CMAKE_HAVE_THREADS_LIBRARY)
+  if(NOT Threads_FOUND)
      CHECK_LIBRARY_EXISTS(${LIBNAME} ${FUNCNAME} "" ${VARNAME})
      if(${VARNAME})
        set(CMAKE_THREAD_LIBS_INIT "-l${LIBNAME}")
@@ -58,6 +59,45 @@ macro(_check_threads_lib LIBNAME FUNCNAME VARNAME)
   endif ()
 endmacro()
 
+# Internal helper macro.
+# Do NOT even think about using it outside of this file!
+macro(_check_pthreads_flag)
+  if(NOT Threads_FOUND)
+    # If we did not found -lpthread, -lpthread, or -lthread, look for -pthread
+    if(NOT DEFINED THREADS_HAVE_PTHREAD_ARG)
+      message(STATUS "Check if compiler accepts -pthread")
+      try_run(THREADS_PTHREAD_ARG THREADS_HAVE_PTHREAD_ARG
+        ${CMAKE_BINARY_DIR}
+        ${CMAKE_CURRENT_LIST_DIR}/CheckForPthreads.c
+        CMAKE_FLAGS -DLINK_LIBRARIES:STRING=-pthread
+        COMPILE_OUTPUT_VARIABLE OUTPUT)
+
+      if(THREADS_HAVE_PTHREAD_ARG)
+        if(THREADS_PTHREAD_ARG STREQUAL "2")
+          set(Threads_FOUND TRUE)
+          message(STATUS "Check if compiler accepts -pthread - yes")
+        else()
+          message(STATUS "Check if compiler accepts -pthread - no")
+          file(APPEND
+            ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeError.log
+            "Determining if compiler accepts -pthread returned ${THREADS_PTHREAD_ARG} instead of 2. The compiler had the following output:\n${OUTPUT}\n\n")
+        endif()
+      else()
+        message(STATUS "Check if compiler accepts -pthread - no")
+        file(APPEND
+          ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeError.log
+          "Determining if compiler accepts -pthread failed with the following output:\n${OUTPUT}\n\n")
+      endif()
+
+    endif()
+
+    if(THREADS_HAVE_PTHREAD_ARG)
+      set(Threads_FOUND TRUE)
+      set(CMAKE_THREAD_LIBS_INIT "-pthread")
+    endif()
+  endif()
+endmacro()
+
 if(CMAKE_HAVE_SPROC_H AND NOT CMAKE_THREAD_PREFER_PTHREAD)
   # We have sproc
   set(CMAKE_USE_SPROC_INIT 1)
@@ -89,41 +129,7 @@ else()
       endif()
     endif()
 
-    if(NOT CMAKE_HAVE_THREADS_LIBRARY)
-      # If we did not found -lpthread, -lpthread, or -lthread, look for -pthread
-      if("x${THREADS_HAVE_PTHREAD_ARG}" STREQUAL "x")
-        message(STATUS "Check if compiler accepts -pthread")
-        try_run(THREADS_PTHREAD_ARG THREADS_HAVE_PTHREAD_ARG
-          ${CMAKE_BINARY_DIR}
-          ${CMAKE_CURRENT_LIST_DIR}/CheckForPthreads.c
-          CMAKE_FLAGS -DLINK_LIBRARIES:STRING=-pthread
-          COMPILE_OUTPUT_VARIABLE OUTPUT)
-
-        if(THREADS_HAVE_PTHREAD_ARG)
-          if(THREADS_PTHREAD_ARG STREQUAL "2")
-            set(Threads_FOUND TRUE)
-            message(STATUS "Check if compiler accepts -pthread - yes")
-          else()
-            message(STATUS "Check if compiler accepts -pthread - no")
-            file(APPEND
-              ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeError.log
-              "Determining if compiler accepts -pthread returned ${THREADS_PTHREAD_ARG} instead of 2. The compiler had the following output:\n${OUTPUT}\n\n")
-          endif()
-        else()
-          message(STATUS "Check if compiler accepts -pthread - no")
-          file(APPEND
-            ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeError.log
-            "Determining if compiler accepts -pthread failed with the following output:\n${OUTPUT}\n\n")
-        endif()
-
-      endif()
-
-      if(THREADS_HAVE_PTHREAD_ARG)
-        set(Threads_FOUND TRUE)
-        set(CMAKE_THREAD_LIBS_INIT "-pthread")
-      endif()
-
-    endif()
+    _check_pthreads_flag()
   endif()
 endif()