I don't have a small simple test case to prove it, but with our app, it seems that if you try to build it in a non-jhbuild environment, but non-default prefix, the -L flags to g-ir-scanner are needed but don't get used.

When building a normal package, /usr/lib will be in the path anyway

In jhbuild, LD_LIBRARY_PATH is set anyway

So this only gets noticed if you build without jhbuild in a nonstandard prefix.

In that case, though, unclear that g-ir-scanner -L is doing anything.

This may be fixed, I thought I saw a related-looking patch go by.