The check seems to be broken, however adjusting L480 in meson.build (https://github.com/irssi/irssi/blob/master/meson.build#L480) to nvlist_create_found = libnv.found() and cc.has_function('nvlist_create', dependencies : libnv) fixes it.

However the build fails shortly after with src/core/meson.build:10:2: ERROR: File fe-capsicum.c does not exist.

Tested using git checkout hash 561619f