gjs fails in gnome3 moduleset but doesn't in gnome-shell moduleset

[Marcel <lists nightsoul org>]

Moin,

I've built gnome-shell successfully for some time now and gjs builds 
fine in that moduleset. However, using the gnome3-moduleset it fails:

checking if SpiderMonkey needs extra compiler flags... yes
checking if -I/usr/include/xulrunner-1.9.2/unstable works... no
configure: error: Unable to determine extra compiler flags needed

When building GS, the configure script concludes that SpiderMonkey 
doesn't need extra compiler flags and the second check therefore isn't 
done at all. Both source/gjs/configure.am scripts don't differ.

That unstable directory indeed doesn't exist (Arch Linux), if I remove 
the "/unstable" part from configure.am and reconfigure, the check seems 
to succeed, but the compiler is missing some glib headers. (Which 
shouldn't happen, too...)

I already built gnome3 from scratch two times and -afc'd gnome-shell, 
nothing changes.
What might be wrong?

Marcel