According to ACE-INSTALL.html there are two ways of building ACE, autoconf based and "traditional" GNU Make. The ebuild is using autoconf based build, but there is a line "emake static_libs=1" that is supposed to work only with GNU Make building.
I'm attaching a proposed patch to allow use-based building of static libraries.
Created attachment 272297 [details, diff]
Enable USE-based building of static libraries
Fixed in 5.8.3, thanks for your contribution
Author: Sergey Popov <email@example.com>
Date: Fri Aug 14 13:36:44 2015 +0300
dev-libs/ace: version bump, enable proper building of static libraries