There have been various reports on this error :
http://stackoverflow.com/questions/15569664/system-missingmethodexception-int32-system-environment-get-currentmanagedthread
https://forum.unity3d.com/threads/monodevelop-build-failed.275506/
http://marcgravell.blogspot.ca/2012/09/iterator-blocks-missing-methods-and-net.html
The stack overflow best describes the issue (and cause), however I can find nowhere a plausible solution if you develop on .Net 4.5, but have customers which have only up to .Net 4.0. Although we target 4.0, VS still uses 4.5 as a base.
Is there any way we can 'force' the compile to 4.0 as we have enterprise clients who's company policy simply do not allow upgrades to 4.5 (or higher) ?
Regards,
Alex