USE_GCC doesn't set rpath

[Yuri]

Current port net-p2p/libtorrent-rasterbar has USES=compiler:c++11-lib 
which in turn sets USE_GCC=4.7+
Maintainer of lang/gccNN says that USE_GCC is supposed to automatically 
set rpath to this particular gcc shared library.
However, this doesn't happen, and qbittorrent fails as a result on 9.2:
$ qbittorrent
/usr/lib/libstdc++.so.6: version GLIBCXX_3.4.15 required by 
/usr/local/lib/libtorrent-rasterbar.so.7 not found

It looks like this automatic setting of rpath is broken.

Yuri