undefined reference to `logf@GLIBCXX_3.4' fix

[coypu%sdf.org@localhost]

If you are seeing this error, which affects only -current, it is because
the libstdc++ configuration was slightly wrong for a few months.
Fixing involved removing a symbol, so you get an undefined reference.

If you see:
/usr/pkgobj/graphics/gimp/work/.buildlink/lib/libexiv2.so: undefined reference to `logf@GLIBCXX_3.4'

Do:
pkg_info -Fe /usr/pkg/lib/libexiv2.so
To find which package is affected by the ABI breakage, and rebuild it to
get correct references.