Although it is possible to give specific compiler instruction, I'd remove the files from the solution and put them somewhere safe in version control.
Having these files in the solution can easily be confusing when browsing the sources; I would not expect to have to check the BuildAction property of C# files to find out whether or not I am looking at something that is part of the build.
I read a lot of code in a source control viewer (looking a delta's and history), compiler options are not very obvious in environments like these.
Put them safe in source control and remove them from the build.