Pulseaudio's configure script documents (-h) only one option for directing the building of the lirc module:
There is no --enable-lirc. However, ebuilds have "$(use_enable lirc)". With USE="lirc" this passes --enable-lirc which apparently (and unexpectedly) results in *disabling* the building of the lirc module.
Furthermore, in my experiments, not passing *any* configure flags wrt lirc lets pulseaudio's configure script autodetect lirc. It then builds the module.
Looks to work on with 4.99.4