Unfortunately Linderdaum doesn't have the licensing I was interested in. It's free for non-commercial use, and available for commercial use at an unspecified price.
"Them?" You are listed as a core developer; perhaps you don't handle the pricing. I will say, when developers list their price as "unspecified," it comes across as a "business to business" proposition. That implies a big contract negotiation, and that it's going to be expensive. As a small indie developer, contacting someone about such an arrangement is not interesting. I'd either be interested in off the shelf, "here's what it is" product terms, or open source as I originally posted.
I find it odd that you are having trouble with building ClanLib on Linux. ClanLib started off being Linux only. In its beginnings it was mostly a 2d library though.
Most of the open source engines have a linux target. Besides the ones already mentioned there is also Crystal Space which is one of the oldest engines available on Linux, but is still actively developed. It looks like it is being included in the 2013 Google Summer of Code. Not sure which OpenGL spec they are up to, but they are OpenGL exclusive so its the focus of what they do.
1) I debugged, submitted patches, or otherwise precipitated them fixing things.
2) I found out they are a strictly hobbyist project, and thus are not of use to me.
This is what happens, I think, when people have a hobbyist mentality to development. They don't realize how little labor they have at their disposal. Years go by and the project drifts in such and such direction, depending on who's actually working on it. Without an effort to attract a critical mass of developers, some things fall by the wayside.ClanLib started off being Linux only.
I noticed the bones of that in their demo code. I'm not sure how far beyond 2D they've moved.In its beginnings it was mostly a 2d library though.
LGPLCrystal Space which is one of the oldest engines available on Linux, but is still actively developed.