graphics/librsvg2: add options, remove libgsf dependency

This is a combination of patches from two PRs, from Andriy Voskoboinyk
and Andrew Romanenko, which have been languishing. Both added a DOCS
option, and Andrew makes Vala optional while Andriy removes an unneeded
dependency on libgsf and gettext.

Note that graphics/librsvg2 is the outdated version, used only on
platforms where the newer librsvg2-rust is not available.

PR: 243071 239346
Reported by: Andrew Romanenkom, Andriy Voskoboinyk