Good news is you can build the .net dll from Visual Studio source code provide at log4Net site https://logging.apache.org/log4net/download_log4net.cgi and choose the right target framework that suits your needs.
Then you can exposed the classes you want as COM classes so C++ can consume them. You'll have to add some interfaces to the code.
This article gives a great explanation "How to call a managed DLL from native Visual C++ code in Visual Studio.NET or in Visual Studio 2005" https://support.microsoft.com/en-ca/kb/828736
Then sign your dll.
sn.exe -k MyKeyFile.SNK
Replace the following lines of code in the AssemblyInfo.cs file
[assembly: ComVisible(false)]
[assembly: AssemblyDelaySign(false)]
[assembly: AssemblyKeyFile("")]
with the following.
[assembly: ComVisible(true)]
[assembly: AssemblyDelaySign(false)]
[assembly: AssemblyKeyFile("..\\..\\MyKeyFile.SNK")]
Press CTRL+SHIFT+B to generate the managed DLL to use in your C++ project.