ビルド前イベントで「特定のファイルが無ければコピーする」を Win:Visual Studio／Mac:Xamarin Studio(Visual Studio for Mac) で行う。< PreBuildEvent > を見てね。

XFGoogleMapSample.csproj

<?xml version="1.0" encoding="utf-8"?>
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    <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
    <Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
    <ProjectGuid>{E5FEBCA7-6127-4002-B7B3-3EE4CC362B25}</ProjectGuid>
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    <UseMSBuildEngine>true</UseMSBuildEngine>
    <OutputType>Library</OutputType>
    <RootNamespace>XFGoogleMapSample</RootNamespace>
    <AssemblyName>XFGoogleMapSample</AssemblyName>
    <TargetFrameworkVersion>v4.5</TargetFrameworkVersion>
    <TargetFrameworkProfile>Profile78</TargetFrameworkProfile>
  </PropertyGroup>
  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
    <DebugSymbols>true</DebugSymbols>
    <DebugType>full</DebugType>
    <Optimize>false</Optimize>
    <OutputPath>bin\Debug</OutputPath>
    <DefineConstants>DEBUG;</DefineConstants>
    <ErrorReport>prompt</ErrorReport>
    <WarningLevel>4</WarningLevel>
    <ConsolePause>false</ConsolePause>
  </PropertyGroup>
  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' ">
    <Optimize>true</Optimize>
    <OutputPath>bin\Release</OutputPath>
    <ErrorReport>prompt</ErrorReport>
    <WarningLevel>4</WarningLevel>
    <ConsolePause>false</ConsolePause>
  </PropertyGroup>
  <ItemGroup>
    <EmbeddedResource Include="App.xaml" />
    <EmbeddedResource Include="MainPage.xaml" />
    <EmbeddedResource Include="BasicMapPage.xaml">
      <Generator>MSBuild:UpdateDesignTimeXaml</Generator>
    </EmbeddedResource>
    <EmbeddedResource Include="PinsPage.xaml">
      <Generator>MSBuild:UpdateDesignTimeXaml</Generator>
    </EmbeddedResource>
    <EmbeddedResource Include="ShapesPage.xaml">
      <Generator>MSBuild:UpdateDesignTimeXaml</Generator>
    </EmbeddedResource>
    <EmbeddedResource Include="TilesPage.xaml">
      <Generator>MSBuild:UpdateDesignTimeXaml</Generator>
    </EmbeddedResource>
    <EmbeddedResource Include="CustomPinsPage.xaml">
      <Generator>MSBuild:UpdateDesignTimeXaml</Generator>
    </EmbeddedResource>
    <EmbeddedResource Include="marker01.png" />
    <EmbeddedResource Include="marker02.png" />
    <EmbeddedResource Include="marker03.png" />
    <EmbeddedResource Include="ShapesWithInitializePage.xaml">
      <Generator>MSBuild:UpdateDesignTimeXaml</Generator>
    </EmbeddedResource>
    <EmbeddedResource Include="BindingPinViewPage.xaml">
      <Generator>MSBuild:UpdateDesignTimeXaml</Generator>
    </EmbeddedResource>
    <EmbeddedResource Include="BindingPinView.xaml">
      <Generator>MSBuild:UpdateDesignTimeXaml</Generator>
    </EmbeddedResource>
    <EmbeddedResource Include="GroundOverlaysPage.xaml">
      <Generator>MSBuild:UpdateDesignTimeXaml</Generator>
    </EmbeddedResource>
  </ItemGroup>
  <ItemGroup>
    <Compile Include="App.xaml.cs">
      <DependentUpon>App.xaml</DependentUpon>
    </Compile>
    <Compile Include="MainPage.xaml.cs">
      <DependentUpon>MainPage.xaml</DependentUpon>
    </Compile>
    <Compile Include="Properties\AssemblyInfo.cs" />
    <Compile Include="BasicMapPage.xaml.cs">
      <DependentUpon>BasicMapPage.xaml</DependentUpon>
    </Compile>
    <Compile Include="PinsPage.xaml.cs">
      <DependentUpon>PinsPage.xaml</DependentUpon>
    </Compile>
    <Compile Include="ShapesPage.xaml.cs">
      <DependentUpon>ShapesPage.xaml</DependentUpon>
    </Compile>
    <Compile Include="TilesPage.xaml.cs">
      <DependentUpon>TilesPage.xaml</DependentUpon>
    </Compile>
    <Compile Include="CustomPinsPage.xaml.cs">
      <DependentUpon>CustomPinsPage.xaml</DependentUpon>
    </Compile>
    <Compile Include="ShapesWithInitializePage.xaml.cs">
      <DependentUpon>ShapesWithInitializePage.xaml</DependentUpon>
    </Compile>
    <Compile Include="BindingPinViewPage.xaml.cs">
      <DependentUpon>BindingPinViewPage.xaml</DependentUpon>
    </Compile>
    <Compile Include="BindingPinView.xaml.cs">
      <DependentUpon>BindingPinView.xaml</DependentUpon>
    </Compile>
    <Compile Include="GroundOverlaysPage.xaml.cs">
      <DependentUpon>GroundOverlaysPage.xaml</DependentUpon>
    </Compile>
    <Compile Include="Extensions\ListExtensions.cs" />
    <Compile Include="Variables.cs" />
  </ItemGroup>
  <ItemGroup>
    <Reference Include="Xamarin.Forms.Core">
      <HintPath>..\packages\Xamarin.Forms.2.3.0.49\lib\portable-win+net45+wp80+win81+wpa81+MonoAndroid10+MonoTouch10+Xamarin.iOS10\Xamarin.Forms.Core.dll</HintPath>
    </Reference>
    <Reference Include="Xamarin.Forms.Xaml">
      <HintPath>..\packages\Xamarin.Forms.2.3.0.49\lib\portable-win+net45+wp80+win81+wpa81+MonoAndroid10+MonoTouch10+Xamarin.iOS10\Xamarin.Forms.Xaml.dll</HintPath>
    </Reference>
    <Reference Include="Xamarin.Forms.Platform">
      <HintPath>..\packages\Xamarin.Forms.2.3.0.49\lib\portable-win+net45+wp80+win81+wpa81+MonoAndroid10+MonoTouch10+Xamarin.iOS10\Xamarin.Forms.Platform.dll</HintPath>
    </Reference>
    <Reference Include="Xamarin.Forms.GoogleMaps">
      <HintPath>..\packages\Xamarin.Forms.GoogleMaps.1.7.1-beta2\lib\portable-net45+wp8+wpa81+win8+MonoAndroid10+MonoTouch10+Xamarin.iOS10+UAP10\Xamarin.Forms.GoogleMaps.dll</HintPath>
    </Reference>
  </ItemGroup>
  <ItemGroup>
    <None Include="packages.config" />
  </ItemGroup>
  <Import Project="$(MSBuildExtensionsPath32)\Microsoft\Portable\$(TargetFrameworkVersion)\Microsoft.Portable.CSharp.targets" />
  <Import Project="..\packages\Xamarin.Forms.2.3.0.49\build\portable-win+net45+wp80+win81+wpa81+MonoAndroid10+MonoTouch10+Xamarin.iOS10\Xamarin.Forms.targets" Condition="Exists('..\packages\Xamarin.Forms.2.3.0.49\build\portable-win+net45+wp80+win81+wpa81+MonoAndroid10+MonoTouch10+Xamarin.iOS10\Xamarin.Forms.targets')" />
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            <DefaultFormat AttributesInNewLine="True" />
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          <TextStylePolicy inheritsSet="null" scope="application/xml" />
          <XmlFormattingPolicy inheritsSet="Mono" inheritsScope="application/xml" scope="application/xml">
            <DefaultFormat AttributesInNewLine="True" />
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  <PropertyGroup Condition=" '$(OS)' == 'Windows_NT' ">
    <PreBuildEvent>if not exist "$(ProjectDir)Variables.cs" copy "$(ProjectDir)Variables_sample.cs" "$(ProjectDir)Variables.cs" >nul</PreBuildEvent>
  </PropertyGroup>
  <PropertyGroup Condition=" '$(OS)' != 'Windows_NT' ">
    <PreBuildEvent>rsync -u "$(ProjectDir)Variables_sample.cs" "$(ProjectDir)Variables.cs"</PreBuildEvent>
  </PropertyGroup>
</Project>