CONFIG::desktop is a compiler option. E.g. this is the line for my debug build, with two such options
mxmlc code/Main.as -o bin/Qyx.swf -debug=true -define=CONFIG::Android,false -define=CONFIG::Debug,true
I don't use FB or FD so don't know where you would add them. If necessary you could add and remove the relevant code manually as you do different builds. Compiler options let you automate this.