Ha! Got it! :D

With joint forces (vekin on #gentoo@irc.freenode.net actually helped) I got the bugger. It's actually not a bug but a sloppily configured install (on my part at least).

You have to locate your Layman's make.conf and include that path into /etc/make.conf. In my case I had to put:

source /usr/portage/local/layman/make.conf

After that it worked like a charm :]

It seems that by default it reads only the /etc/make.conf and if you have overlays handled by Layman it doesn't look into those settings. So by adding that line you tell it to take into consideration Layman's settings as well.

I hope this solves your problem as well.