Created attachment 144855 [details] adds glib20 to USE_GNOME rrdtool links against glib, but glib isn't listed as a dependency in the port. Most people won't notice, because cairo depends on glib and therefore glib is an implicity dependency of rrdtool. However, it's possible to remove the cairo dependency (I'll submit that as another PR). In that case, it's important to list glib in the Makefile.
over to maintainer
I'm working on the rrdtool port already, and have already patched this. Will commit together with other changes.
Fixed in commit r362726.