system/build-tools: should only pull in man-pages when `docs` is installed
The docs
metapackage installs -doc
packages for all the installed packages that have documentation.
man-pages
is a pretty hefty load and most builders don't actually need it. It is, however, helpful for end users.
To that end, build-tools
should really only depend on build-tools
when docs
is installed, the same way other -doc
(and -openrc
, etc) packages work. This saves time and space on builder systems, and any other non-interactive systems used by us and end users that need build-tools
.