Dependency handling in pkgsrc is rather complex task. There exist some cases (TeX packages, Perl packages) where it is hard to find build dependencies precisely and the whole thing is handled conservatively. E.g. the whole TeXLive meta-package is declared a build dependency even when rather small fraction of it is used actually. Another case is stale heavy dependency which is no longer required but still listed as prerequisite.
It would be nice to have a tool (or a set of them, if necessary) to detect which installed packages, libraries or tools were actually used to build new package. Ideally, the tool should report files used during configure, build, and test stages, and packages these files are provided by.