I got this bug and as I don't know if this is correct what the user says (no 
idea about smp), I'm posting this here for comments:

https://bugs.gentoo.org/show_bug.cgi?id=224729

cite bug report:
I added "threads" to make.conf a while ago on my SMP machine. Now i merged 
gimp
and found that it uses the flag "smp" for the same purpose. At least i think
its for the same purpose.
This is confusing and forces users to maintain local flags for same apps. I
would suggest that all ebuilds using the smp-flag switch to threads if there 
is no good reason to have a separate USE flag.
[...]
This is a recent list of ebuilds using the local flag "smp"

dev-lang/erlang/erlang-12.2.2.ebuild
games-fps/alephone/alephone-20071103.ebuild
media-gfx/gimp/gimp-2.3.19.ebuild
media-gfx/gimp/gimp-2.4.2.ebuild
media-gfx/gimp/gimp-2.4.5.ebuild
media-gfx/gimp/gimp-9999.ebuild
media-plugins/gimp-greycstoration/gimp-greycstoration-2.8.ebuild
sys-cluster/charm/charm-5.9.ebuild
sys-cluster/openmpi/openmpi-1.2.5.ebuild
sys-cluster/openmpi/openmpi-1.2.6.ebuild
-- 
Hanno Böck		Blog:   http://www.hboeck.de/
GPG: 3DBD3B20		Jabber: hanno@...