Jared,

> Sounds familiar. In xorg.conf, you want:
> 
>   Section "ServerFlags"
>       Option "NoTrapSignals"
>   EndSection
> 
> This way, if you run it as root, it will dump core rather than trying to 
> shutdown cleanly :)
> 
> I'm willing to bet that your Xorg binary isn't linked against libpthread 
> -- is this correct? If so you need to relink it for AIGLX to work.

Yep, you were right and twice right. With an extra LDFLAGS=-lpthread all 
is fine, except that I get this:

(WW) AIGLX: 3D driver claims to not support visual 0x23
(WW) AIGLX: 3D driver claims to not support visual 0x24
(WW) AIGLX: 3D driver claims to not support visual 0x25
(WW) AIGLX: 3D driver claims to not support visual 0x26
(WW) AIGLX: 3D driver claims to not support visual 0x27
(WW) AIGLX: 3D driver claims to not support visual 0x28
(WW) AIGLX: 3D driver claims to not support visual 0x29
(WW) AIGLX: 3D driver claims to not support visual 0x2a
(WW) AIGLX: 3D driver claims to not support visual 0x2b
(WW) AIGLX: 3D driver claims to not support visual 0x2c
(WW) AIGLX: 3D driver claims to not support visual 0x2d
(WW) AIGLX: 3D driver claims to not support visual 0x2e
(WW) AIGLX: 3D driver claims to not support visual 0x2f
(WW) AIGLX: 3D driver claims to not support visual 0x30
(WW) AIGLX: 3D driver claims to not support visual 0x31
(WW) AIGLX: 3D driver claims to not support visual 0x32
(II) AIGLX: Loaded and initialized /usr/pkg/lib/dri/radeon_dri.so
(II) GLX: Initialized DRI GL provider for screen 0

Which I assume means that no 3D-acceleration is available anyway.

Thanks for your help and all, sorry for the noise.
Vincent