The syntax "pkg?vpkg" can be used in hostmakedepends, makedepends and depends;
but the "virtual" keyword can be declared specifically for runtime dependencies
(depends).
Run-time dependencies declared via "${depends}" are now also part of the
build process, but those are not installed to the master directory; rather
those dependencies are only checked if a binary package exists in a
local repository to satisfy the required version.
Thanks to @dominikh for idea and suggestions.
The new xbps-src configuration file is `etc/conf` where you can
add your local overrides from defaults set via `etc/defaults.conf`.
To use this xbps-src, run these steps:
$ make
$ sudo make setup (to make chroot helper setgid)
$ ./xbps-src ...
