allow Ctrl+Break in the shell. Works on Windows too.

GF.cabal

@@ -11,6 +11,9 @@ flag readline
   Description: Enable Readline in the shell
   Default:     True
 
+flag interrupt
+  Description: Enable Ctrl+Break in the shell
+  Default:     True
 
 library
   build-depends: base,
@@ -174,3 +177,9 @@ executable gf3
     other-modules: GF.System.UseReadline
   else
     other-modules: GF.System.NoReadline
+  
+  if flag(interrupt)
+    ghc-options: -DUSE_INTERRUPT
+    other-modules: GF.System.UseSignal
+  else
+    other-modules: GF.System.NoSignal