Allow to disable self-upgrade functionality wrt #305

ghcup.cabal

@@ -48,6 +48,13 @@ flag no-exe
   default:     False
   manual:      True
 
+flag disable-upgrade
+  description:
+    Build the brick powered tui (ghcup tui). This is disabled on windows.
+
+  default:     False
+  manual:      True
+
 library
   exposed-modules:
     GHCup
@@ -195,7 +202,6 @@ executable ghcup
     GHCup.OptParse.Set
     GHCup.OptParse.ToolRequirements
     GHCup.OptParse.UnSet
-    GHCup.OptParse.Upgrade
     GHCup.OptParse.Whereis
 
   hs-source-dirs:     app/ghcup
@@ -262,6 +268,12 @@ executable ghcup
   if flag(no-exe)
     buildable: False
 
+  if (flag(disable-upgrade))
+    cpp-options:   -DDISABLE_UPGRADE
+  else
+    other-modules:
+        GHCup.OptParse.Upgrade
+
 test-suite ghcup-test
   type:               exitcode-stdio-1.0
   main-is:            Main.hs