steam-launcher wants the multilib use flag on gcc. It seems gcc-4.7.1 does not have this use flag Reproducible: Always
Hm, gcc-4.7.1 as well as gcc-4.7.3-r1 have a multilib use flag on my system. Do you use the multilib profile?
Indeed, I'm using the no multilib profile. I eselected the multilib profile and it failed to update my system. glibc and gcc-4.7 failed to be migrated from no multilib to multilib.
According to https://wiki.gentoo.org/wiki/AMD64/FAQ#What_is_multilib_and_how_can_I_use_it.3F it is not possible to do such a switch. I've always used the multilib profile, so I don't know why this is not possible, sorry.