Allow overriding NUM_THREADS

cmake/system.cmake

@@ -94,16 +94,17 @@ if (NOT CMAKE_CROSSCOMPILING)
     ProcessorCount(NUM_CORES)
   endif()
 
+endif()
+
+if (NOT DEFINED NUM_THREADS)
   if (NOT NUM_CORES EQUAL 0)
     # HT?
     set(NUM_THREADS ${NUM_CORES})
+  else ()
+    set(NUM_THREADS 0)
   endif ()
 endif()
 
-if (NOT DEFINED NUM_THREADS)
-  set(NUM_THREADS 0)
-endif()
-
 if (${NUM_THREADS} LESS 2)
   set(USE_THREAD 0)
 elseif(NOT DEFINED USE_THREAD)