Permanent global pkg build options can be defined via 'XBPS_PKG_OPTIONS' in etc/conf.
Permament per-pkg build options can be defined via 'XBPS_PKG_OPTIONS_<pkgname>' in etc/conf.
Close GH #452.
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 ...