make --define:noSignalHandler compile again

lib/system.nim

@@ -3463,7 +3463,7 @@ when not defined(JS): #and not defined(nimscript):
       ## allows you to override the behaviour of your application when CTRL+C
       ## is pressed. Only one such hook is supported.
 
-    when not defined(useNimRtl):
+    when not defined(noSignalHandler) and not defined(useNimRtl):
       proc unsetControlCHook*()
         ## reverts a call to setControlCHook
 

lib/system/excpt.nim

@@ -537,7 +537,7 @@ proc setControlCHook(hook: proc () {.noconv.}) =
   type SignalHandler = proc (sign: cint) {.noconv, benign.}
   c_signal(SIGINT, cast[SignalHandler](hook))
 
-when not defined(useNimRtl):
+when not defined(noSignalHandler) and not defined(useNimRtl):
   proc unsetControlCHook() =
     # proc to unset a hook set by setControlCHook
     c_signal(SIGINT, signalHandler)