> Following the "adding module ..." line, nginx configure calls the
> "config" script from the module directory. And since there is no
> further output, it hangs somewhere in the config script of the
> passenger module.

You're right, I spend a good part of the day to track this down and figured out that the Passenger config script hangs whenever it invokes Ruby. And this happens only if Ruby is using hardened jemalloc. Seems to be related to this problem:

https://bugs.gentoo.org/617518