Make sure to add the compat, and then remove in the clean

build.cake

@@ -277,10 +277,59 @@ var VisualStudioPathFixup = new Action (() => {
     }
 });
 
+var InjectCompatibilityExternals = new Action<bool> ((inject) => {
+    // some methods don't yet exist, so we must add the compat layer to them.
+    // we need this as we can't modify the third party files
+    // all we do is insert our header before all the others
+    var compatHeader = "native-builds/src/WinRTCompat.h";
+    var compatSource = "native-builds/src/WinRTCompat.c";
+    var files = new Dictionary<FilePath, string> { 
+        { "skia/third_party/externals/dng_sdk/source/dng_string.cpp", "#if qWinOS" },
+        { "skia/third_party/externals/dng_sdk/source/dng_utils.cpp", "#if qWinOS" },
+        { "skia/third_party/externals/dng_sdk/source/dng_pthread.cpp", "#if qWinOS" },
+        { "skia/third_party/externals/zlib/deflate.c", "#include <assert.h>" },
+        { "skia/third_party/externals/libpng/pngpriv.h", "#  include <windows.h>  /* defines _WINDOWS_ macro */" },
+    };
+    foreach (var filePair in files) {
+        var file = filePair.Key;
+        var root = string.Join ("/", file.GetDirectory().Segments.Select (x => ".."));
+        var include = "#include \"" + root + "/" + compatHeader + "\"";
+        
+        var contents = FileReadLines (file).ToList ();
+        var index = contents.IndexOf (include);
+        if (index == -1 && inject) {
+            if (string.IsNullOrEmpty (filePair.Value)) {
+                contents.Insert (0, include);
+            } else {
+                contents.Insert (contents.IndexOf (filePair.Value), include);
+            }
+            FileWriteLines (file, contents.ToArray ());
+        } else if (index != -1 && !inject) {
+            int idx = 0;
+            if (string.IsNullOrEmpty (filePair.Value)) {
+                idx = 0;
+            } else {
+                idx = contents.IndexOf (filePair.Value) - 1;
+            }
+            if (contents [idx] == include) {
+                contents.RemoveAt (idx);
+            }
+            FileWriteLines (file, contents.ToArray ());
+        }
+    }
+});
+
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 // EXTERNALS - the native C and C++ libraries
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
+Task ("externals-base")
+    .Does (() => 
+{
+    // add compatibility methods to skia externals
+    InjectCompatibilityExternals (true);
+});
+
 // this builds all the externals
 Task ("externals")
     .IsDependentOn ("externals-genapi")
@@ -347,6 +396,7 @@ Task ("externals-genapi")
 });
 // this builds the native C and C++ externals for Windows
 Task ("externals-windows")
+    .IsDependentOn ("externals-base")
     .WithCriteria (IsRunningOnWindows ())
     .WithCriteria (
         !FileExists ("native-builds/lib/windows/x86/libSkiaSharp.dll") ||
@@ -374,6 +424,7 @@ Task ("externals-windows")
 });
 // this builds the native C and C++ externals for Windows UWP
 Task ("externals-uwp")
+    .IsDependentOn ("externals-base")
     .WithCriteria (IsRunningOnWindows ())
     .WithCriteria (
         !FileExists ("native-builds/lib/uwp/ARM/libSkiaSharp.dll") ||
@@ -386,35 +437,6 @@ Task ("externals-uwp")
         // TODO: the stuff in this block must be moved into the gyp files !!
         //
 
-        // some methods don't yet exist, so we must add the compat layer to them.
-        // we need this as we can't modify the third party files
-        // all we do is insert our header before all the others
-        var compatHeader = "native-builds/src/WinRTCompat.h";
-        var compatSource = "native-builds/src/WinRTCompat.c";
-        var files = new Dictionary<FilePath, string> { 
-            { "skia/third_party/externals/dng_sdk/source/dng_string.cpp", "#if qWinOS" },
-            { "skia/third_party/externals/dng_sdk/source/dng_utils.cpp", "#if qWinOS" },
-            { "skia/third_party/externals/dng_sdk/source/dng_pthread.cpp", "#if qWinOS" },
-            { "skia/third_party/externals/zlib/deflate.c", "#include <assert.h>" },
-            { "skia/third_party/externals/libpng/pngpriv.h", "#  include <windows.h>  /* defines _WINDOWS_ macro */" },
-        };
-        foreach (var filePair in files) {
-            var file = filePair.Key;
-            var root = string.Join ("/", file.GetDirectory().Segments.Select (x => ".."));
-            var include = "#include \"" + root + "/" + compatHeader + "\"";
-            
-            var contents = FileReadLines (file).ToList ();
-            var index = contents.IndexOf (include);
-            if (index == -1) {
-                if (string.IsNullOrEmpty (filePair.Value)) {
-                    contents.Insert (0, include);
-                } else {
-                    contents.Insert (contents.IndexOf (filePair.Value), include);
-                }
-                FileWriteLines (file, contents.ToArray ());
-            }
-        }
-        
         var projectFile = MakeAbsolute (projectFilePath).FullPath;
         var xdoc = XDocument.Load (projectFile);
         
@@ -534,6 +556,7 @@ Task ("externals-uwp")
 });
 // this builds the native C and C++ externals for Mac OS X
 Task ("externals-osx")
+    .IsDependentOn ("externals-base")
     .WithCriteria (IsRunningOnUnix ())
     .WithCriteria (
         !FileExists ("native-builds/lib/osx/libSkiaSharp.dylib"))
@@ -570,6 +593,7 @@ Task ("externals-osx")
 });
 // this builds the native C and C++ externals for iOS
 Task ("externals-ios")
+    .IsDependentOn ("externals-base")
     .WithCriteria (IsRunningOnUnix ())
     .WithCriteria (
         !FileExists ("native-builds/lib/ios/libSkiaSharp.framework/libSkiaSharp"))
@@ -613,6 +637,7 @@ Task ("externals-ios")
 });
 // this builds the native C and C++ externals for tvOS
 Task ("externals-tvos")
+    .IsDependentOn ("externals-base")
     .WithCriteria (IsRunningOnUnix ())
     .WithCriteria (
         !FileExists ("native-builds/lib/tvos/libSkiaSharp.framework/libSkiaSharp"))
@@ -659,6 +684,7 @@ Task ("externals-tvos")
 });
 // this builds the native C and C++ externals for Android
 Task ("externals-android")
+    .IsDependentOn ("externals-base")
     .WithCriteria (IsRunningOnUnix ())
     .WithCriteria (
         !FileExists ("native-builds/lib/android/x86/libSkiaSharp.so") ||
@@ -984,6 +1010,9 @@ Task ("clean-externals").Does (() =>
     // windows
     CleanDirectories ("native-builds/libSkiaSharp_windows/Release");
     CleanDirectories ("native-builds/libSkiaSharp_windows/x64/Release");
+    
+    // remove compatibility
+    InjectCompatibilityExternals (false);
 });
 
 ////////////////////////////////////////////////////////////////////////////////////////////////////