If you're having troubles getting Portage to build packages due to SELinux, then the reason usually is that it is unable to transition to the proper portage domains. You'll get a nice OSError back with an ugly backtrace, saying somewhere that "setexeccon" is misbehaving.
Now, the real issue (not being able to transition) means that the current domain you are in (check id -Z) has no right to transition to the portage_fetch_t, portage_t or portage_sandbox_t domains. You can verify that with seinfo:
~# id -Z unconfined_u:unconfined_r:unconfined_t ~# seinfo -runconfined_r -x | grep portage
The above example shows it for the unconfined_t domain, but the same is true if your current domain is a more illogical local_login_t (hint: check your PAM settings) or initrc_t. Now, if you want to fix these things, we will eventually ask you to reemerge some things - which was the first reason why you came asking how to fix things.
There are two ways to handle this situation: the proper way (disabling SELinux and reenabling later) or the ugly way (hack Portage to ignore).
In the first way, you need to edit /etc/selinux/config, set SELINUX=disabled, reboot, emerge whatever you need, edit the file again restoring SELINUX to what you had before, reboot, relabel your entire filesystem (rlpkg -a -r) and perhaps even reboot again.
In the second method, edit /usr/lib(64)/portage/pym/portage/_selinux.py and go to line 79. It reads:
if selinux.setexeccon(ctx) < 0:
Comment out that line (so it isn't lost) and substitute it with
if selinux.setexeccon("\n") < 0:
Now you should be able to install software without hitting the error. But note that this is only to help you fix the real problem as we're circumventing SELinux integration in Portage a bit.