Explicit error when net35-cf is unlikely to work.

Not sure if there's a nicer way to do this.

Author: airbreather

GeoAPI/GeoAPI.csproj

@@ -147,7 +147,7 @@
     <None Include="$(SolutionDir)geoapi.snk" />
   </ItemGroup>
 
-  <Target Name="BeforeBuild">
-    <Message Text="Building for '$(TargetFramework)'" />
+  <Target Name="CheckNet35CompactFramework" BeforeTargets="ResolveAssemblyReferences">
+    <Error Condition=" '$(TargetFramework)' == 'net35-cf' And !Exists('$(FrameworkPathOverride)\mscorlib.dll') " Text=".NET 3.5 Compact Framework reference assemblies must be installed.  Expected to see $(FrameworkPathOverride)\mscorlib.dll." />
   </Target>
 </Project>