databases/rrdtool: Make groff a conditional dependency

[koobs]

Summary

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:	D14732
MFH:			2018Q3

Note: portlint warnings not addressed in this changeset, will (should) be addressed in its own separate changeset

Test Plan

• portlint: OK (0 warning delta, separate logical changes)
• testport: OK (poudriere, DOCS off)