cmake: Allow thread checks to fail and fall back to nothreads

Closes: #2155


git-svn-id: https://svn.apache.org/repos/asf/xerces/c/trunk@1843653 13f79535-47bb-0310-9956-ffa450edef68

cmake/XercesMutexMgrSelection.cmake

@@ -19,11 +19,15 @@
 
 # mutexmgr selection
 
-option(threads "Threading support" ON)
+set(xerces_thread_default ON)
+find_package(Threads)
+if(NOT TARGET Threads::Threads)
+  set(xerces_thread_default ON)
+endif()
 
-include(CheckCXXSourceCompiles)
+option(threads "Threading support" ${xerces_thread_default})
 
-find_package(Threads REQUIRED)
+include(CheckCXXSourceCompiles)
 
 function(thread_test outvar)
   set(CMAKE_REQUIRED_LIBRARIES ${CMAKE_REQUIRED_LIBRARIES} ${CMAKE_THREAD_LIBS_INIT})