Unfortunately, XIDE hides the .NET Framework selection from the user and uses the newest one available on the developers machine, but fortunately it is possible to change - maybe to build an executable that runs on Windows XP where only .NET 4.0 is available.
Simply follow these steps:
/nostdlib
(it akes the compiler not automatically add a reference to mscorlib.dll)C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework
. Just pick the folder of the framework version that you want to use and add references to those files, including mscorlib.dll.Maybe somewhere in the future there will be a plugin for this if enough people asks for it.