Attempt 2 to address issues with pthreads needed only for certain systems

Author: rcarson3

cmake/thirdpartylibraries/SetupThirdPartyLibraries.cmake

@@ -204,10 +204,8 @@ if(UNIX AND NOT APPLE)
         endif()
     else()
         message(STATUS "  Result: pthread implicitly linked (no action needed)")
-        set(Threads_FOUND TRUE)
     endif()
 
 elseif(APPLE)
-    set(Threads_FOUND TRUE)
     message(STATUS "Threads support built-in on macOS")
 endif()