I dont have Eclipse so cant try this out, so I will suggest it instead. Can you try changing the "Configuration" at the top, from "Debug" to another option? I imagine the other option will be "Release". Does it re-enable the selection when it is set to "Release" (or other build configuration options it gives)?
EDIT: I imagine (or hope!) you have searched around for this, but here is a (somewhat random) page I found explaining the window you're showing:
http://www.linuxtopia.org/online_boo...prop_build.htm. Is there something in there that you're missing? Do you have to create/modify a variable or something? Etc.