Fix CMake cross-compiling

Without specifying thread count, NUM_THREADS would not be defined and CMake would fail.
This is because core count cannot be determined when cross-compiling.

cmake/system.cmake

@@ -97,7 +97,7 @@ if (NOT CMAKE_CROSSCOMPILING)
 endif()
 
 if (NOT DEFINED NUM_THREADS)
-  if (NOT NUM_CORES EQUAL 0)
+  if (DEFINED NUM_CORES AND NOT NUM_CORES EQUAL 0)
     # HT?
     set(NUM_THREADS ${NUM_CORES})
   else ()