GC: added static cycleGC checks

lib/system/gc.nim

@@ -173,6 +173,7 @@ proc rtlAddCycleRoot(c: PCell) {.rtl, inl.} =
   # we MUST access gch as a global here, because this crosses DLL boundaries!
   when hasThreadSupport and hasSharedHeap:
     AcquireSys(HeapLock)
+  when cycleGC:
     if c.color != rcPurple:
       c.setColor(rcPurple)
       incl(gch.cycleRoots, c)
@@ -811,6 +812,7 @@ proc CollectZCT(gch: var TGcHeap): bool =
       # as this might be too slow.
       # In any case, it should be removed from the ZCT. But not
       # freed. **KEEP THIS IN MIND WHEN MAKING THIS INCREMENTAL!**
+      when cycleGC:
         if canBeCycleRoot(c): excl(gch.cycleRoots, c)
       when logGC: writeCell("zct dealloc cell", c)
       gcTrace(c, csZctFreed)