FindThreads: avoid failing in AIX when using -D_XOPEN_SOURCE=500

Current FindThreads fails in AIX (tested on 7.1 and 7.2) when using
additional compilation flags (ex: -D_XOPEN_SOURCE=500) because the
linker might need additional flags to link and those aren't yet known
by the time check_include("pthread.h") is run (which is also the first
test).

Remove the check for it and all supporting code and rely instead on
the compilation test that will be done later to find the correct
syntax to use, and that confirms it exists implicitly.

Modules/FindThreads.cmake

@@ -156,41 +156,27 @@ macro(_threads_check_flag_pthread)
   endif()
 endmacro()
 
-# Do we have pthreads?
-if(CMAKE_C_COMPILER_LOADED)
-  CHECK_INCLUDE_FILE("pthread.h" CMAKE_HAVE_PTHREAD_H)
-else()
-  CHECK_INCLUDE_FILE_CXX("pthread.h" CMAKE_HAVE_PTHREAD_H)
+# Check if pthread functions are in normal C library.
+# We list some pthread functions in PTHREAD_C_CXX_TEST_SOURCE test code.
+# If the pthread functions already exist in C library, we could just use
+# them instead of linking to the additional pthread library.
+_threads_check_libc()
+
+# Check for -pthread first if enabled. This is the recommended
+# way, but not backwards compatible as one must also pass -pthread
+# as compiler flag then.
+if (THREADS_PREFER_PTHREAD_FLAG)
+  _threads_check_flag_pthread()
+endif ()
+
+if(CMAKE_SYSTEM MATCHES "GHS-MULTI")
+  _threads_check_lib(posix pthread_create CMAKE_HAVE_PTHREADS_CREATE)
 endif()
+_threads_check_lib(pthreads pthread_create CMAKE_HAVE_PTHREADS_CREATE)
+_threads_check_lib(pthread  pthread_create CMAKE_HAVE_PTHREAD_CREATE)
 
-if(CMAKE_HAVE_PTHREAD_H)
-  #
-  # We have pthread.h
-  # Let's check for the library now.
-  #
-
-  # Check if pthread functions are in normal C library.
-  # We list some pthread functions in PTHREAD_C_CXX_TEST_SOURCE test code.
-  # If the pthread functions already exist in C library, we could just use
-  # them instead of linking to the additional pthread library.
-  _threads_check_libc()
-
-  # Check for -pthread first if enabled. This is the recommended
-  # way, but not backwards compatible as one must also pass -pthread
-  # as compiler flag then.
-  if (THREADS_PREFER_PTHREAD_FLAG)
-    _threads_check_flag_pthread()
-  endif ()
-
-  if(CMAKE_SYSTEM MATCHES "GHS-MULTI")
-    _threads_check_lib(posix pthread_create CMAKE_HAVE_PTHREADS_CREATE)
-  endif()
-  _threads_check_lib(pthreads pthread_create CMAKE_HAVE_PTHREADS_CREATE)
-  _threads_check_lib(pthread  pthread_create CMAKE_HAVE_PTHREAD_CREATE)
-
-  if (NOT THREADS_PREFER_PTHREAD_FLAG)
-    _threads_check_flag_pthread()
-  endif()
+if (NOT THREADS_PREFER_PTHREAD_FLAG)
+  _threads_check_flag_pthread()
 endif()
 
 if(CMAKE_THREAD_LIBS_INIT OR CMAKE_HAVE_LIBC_PTHREAD)