Solve build warnings

[haf]

    /Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/Microsoft SDKs/F#/3.0/Framework/v4.0/Microsoft.FSharp.Targets:  warning : A circular reference was found involving the import of '/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/4.0/Microsoft.FSharp.Targets'. It was earlier imported by '/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild/Microsoft/VisualStudio/v/FSharp/Microsoft.FSharp.Targets'. Only the first import of this file will be used, ignoring others.
    /Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/Microsoft SDKs/F#/3.0/Framework/v4.0/Microsoft.FSharp.Targets:  warning : A circular reference was found involving the import of '/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/4.0/Microsoft.FSharp.Targets'. It was earlier imported by '/Library/Frameworks/Mono.framework/Versions/3.12.0/lib/mono/xbuild/Microsoft/VisualStudio/v/FSharp/Microsoft.FSharp.Targets'. Only the first import of this file will be used, ignoring others.