Bug 1443682 - Hide the use of OptionalExtensions behind an ifdef. r=mhowell

MozReview-Commit-ID: 8jBeNuMlaYj

--HG--
extra : transplant_source : Eb%C74%0D%AC%DDpb%EBK%83%8F9W%B7%80%87%97%E0

toolkit/mozapps/installer/windows/nsis/common.nsh

@@ -5099,12 +5099,14 @@
               ${ElevateUAC}
             ${EndIf}
 
-            ReadINIStr $R8 $R7 "Install" "OptionalExtensions"
-            ${If} $R8 == "false"
-              StrCpy $InstallOptionalExtensions "0"
-            ${Else}
-              StrCpy $InstallOptionalExtensions "1"
-            ${EndIf}
+            !ifdef MOZ_OPTIONAL_EXTENSIONS
+              ReadINIStr $R8 $R7 "Install" "OptionalExtensions"
+              ${If} $R8 == "false"
+                StrCpy $InstallOptionalExtensions "0"
+              ${Else}
+                StrCpy $InstallOptionalExtensions "1"
+              ${EndIf}
+            !endif
 
             !ifndef NO_STARTMENU_DIR
               ReadINIStr $R8 $R7 "Install" "StartMenuDirectoryName"