I really need some help as I have been trying to get this working for way to long. I got Log4Net compiling and working great on the editor, and on PC/Mac/L build. I now need to make sure that the code is not compile in the WebGL build. For starters because it uses System.Configuration which WebGL does not like, and also because we don’t want additional log code in WebGL.
So,
Can someone give me some ideas of how to do this?
I stupidly figured that if the code was not referenced it would not be built. That is not the case. Then I figured I would but it in the editor folder and have it only compile in editor mode. Problem is I can’t call the code from outside the editor. So what else I can do? I hear of the .rsc files but there is no documentation on how to used them other than to set global defines.
The Log4Net comprises of some code (that I can pack up in a .dll again) I had to make some changes. And then a wrapper file, that used global defines to instantiate the logger or use the simple Debug.log calls.
ANY help please? … 
Yep, I saw that. But I am trying to exclude a set of files.
Ohhh, I am so close. I will answer my own problem in case someone needs this.
If we click on the .dll (so yes the code needs to be in a dll) we can assign Plugin Settings and tell Unity what platform to used the .dll on See Unity - Manual: Import and configure plug-ins
So PERFECT! 
That solved my problem of how to exclude the Log4Net.dll from the WebGL build.
Now I have other problems related to defines, specifically for the editor compilation. We can define defines for different compilations. PC/MAC/Linux and WebGL for example! So far so great, But what about for the Editor compilation. I would like to define LOG4NET_LOG for the editor version and the PC/MAC/Linux versions ONLY ! But I don’t see a way to declare defines for the EDITOR! … I don’t want a GLOBAL in the .rsc file because I don’t want it defined for WEBGL!!
Help?
Sooooo close.