Proposed commit log:
```
databases/rrdtool: Make groff a conditional dependency
groff is required by rrdtool to format documentation, which
is only required when documentation is being built.
Accordingly, make groff conditional on the existing DOCS option.
This precludes building a litany of dependencies, mostly due to groff requiring ghostscript.
Approved by: zeising (maintainer)
Differential_Revision: DXXXX
MFH: 2018Q3
```