I suggest this as default in gentoo: export PKG_CONFIG_PATH=/usr/local/lib/pkgconfig/ The user will possibly install additional libraries into /usr/local/lib and will need it anyway. I can't imagine anything that could be harmed by setting this.
We don't ever install anything into /usr/local, if user installs something there, it's his responsibility to take care of PKG_CONFIG_PATH and whatnot...
I suggest to add it even if it's the user's responsibility - it will save lots of users some time.