GenerateResxSource produces C# code with build errors on .NET Framework 3.0/3.5

[DustinCampbell]

GenerateResxSource produces code with [MethodImpl(MethodImplOptions.AggresiveInlining)], which (IIRC) was not available until .NET 4.5. The generated code surrounds the attribute with #if !NET20, but that returns true if compiling for net35.