Update build.md

Documentation/build.md

@@ -30,7 +30,7 @@ The build process needs [Xcode](https://developer.apple.com/xcode/) to build the
 
 Linux operating systems ship with their own respective package managers however we will use [Homebrew](https://brew.sh/) to manage packages on macOS.  To install follow the instructions [here](https://docs.brew.sh/Installation).
 
-###  Install CastXML
+###  Install CastXML (pre Nov 2020)
 
 Avalonia requires [CastXML](https://github.com/CastXML/CastXML) for XML processing during the build process.  The easiest way to install this is via the operating system's package managers, such as below.
 
@@ -61,6 +61,7 @@ git submodule update --init --recursive
 ### Build native libraries (macOS only)
 
 On macOS it is necessary to build and manually install the respective native libraries using [Xcode](https://developer.apple.com/xcode/).  The steps to get this working correctly are:
+- (for revisions after 2 Nov 2020) Run `./build.sh GenerateCppHeaders` to generate `avalonia-native.h` from `avn.idl`
 - Navigate to the Avalonia/native/Avalonia.Native/src/OSX folder and open the `Avalonia.Native.OSX.xcodeproj` project
 - Build the library via the Product->Build menu.  This will generate binaries in your local path under ~/Library/Developer/Xcode/DerivedData/Avalonia.Native.OSX-*guid* where "guid" is uniquely generated every time you build.
 - Manually install the native library by copying it from the build artifacts folder into the shared dynamic library path: