This might be obvious to some people. You’d think it would have been obvious to me, but it wasn’t. So just in case somebody runs into the same issue:
If by chance you want to use the Vala programming language, remember to install glib2_devel manually alongside the compiler, otherwise you won’t be able to build anything at all. In Debian for example the equivalent package libglib2.0-dev is a dependency for valac, so you can’t forget it, but in Haiku you can!
Hope this spares someone some headaches. Thanks for reading.
Dunno, it worked for me after installing glib2_devel, so nothing seems actually broken.
Upon re-reading my old notes, it appears it’s possible to use Vala without the GObject runtime, so not making the latter a hard dependency is technically correct. Guess the Debian packagers were more focused on typical usage.