While pkg_comp will automatically generate a pkg_summary.gz for locally
built packages at the given path - namely /usr/pkgsrc/packags/All - the
same does not apply for packages generated by running make in the pkgsrc
tree. If you don't know what I'm talking about, see pkg_summary(5) man.   

On my Slackware desktop and my Solaris VM I usually build everything
from source. With a i7 7000 and 16 Gb of physical memroy, this is
acceptable. Not that I'm given much choice: would you use Joyent's packages
built built against EPEL7 on Slackware 15?

Even while compiling from source you may still want to use pkgin to
manage your repo and handle dependencies. A couple of common uses:

- reinstall previously built an later on deinstalled packages

- autoremove unused dependencies

So, this is the script I use to generate a local pkg_summary.

pkg_path="/usr/pkgsrc/packages/All"
sum="/tmp/pkg_summary.gz"
if [ -d $pkg_path ]; then
	( cd $pkg_path ; pkg_info -X *.tgz | gzip -9 > $sum )
fi
mv -v $sum $pkg_path

Place it somewhere in root's PATH (or use sudo -E).

Now edit /usr/pkg/etc/pkgin/repositories.conf to include 

file:///usr/pkgsrc/packages/All

As first or sole repository.

Finally, update your package repo: 

~ $ pkgin -f update
reading local summary...
processing local summary...
reading local summary...
processing local summary...
processing remote summary (file:///usr/pkgsrc/packages/All)...
pkg_summary.gz                   100%   98KB  49.1KB/s   00:02 

And you're ready to go.