The current lirc 0.9.4a ebuild has the use flag 'iguanair' which only adds a dependency on app-misc/iguanaIR and the use flag 'irman'. That would all that is needed for older lirc ebuilds (along with LIRC_DEVICES='iguanair' being set) but in newer lirc versions, the iguanair lirc driver has been moved outside of the lirc sources to https://github.com/iguanaworks/iguanair-lirc. You can also get the driver in the lirc sources until the next release of lirc when it will be removed. Either way it's obtained, the driver is not built by default. Also, I don't know why the irman use flag is required in the ebuild as I can't find anything that says it's needed for the iguanair driver. This lirc driver does still require the app-misc/iguanaIR package to be installed to function. The app-misc/iguanaIR package however can function without lirc or this driver installed. Because of that, I think a new package will probably need to be made just to build the 'iguanair' lirc driver. Then the lirc 0.9.4a ebuild use flag 'iguanair' can require that new package to be installed after the lirc package is installed.
0.10.0 non longer contains it
