[Shotwell] Shotwell - 0.12.3 build errors

[Brad Hermanson]

I'm the maintainer of the Shotwell SlackBuild for Slackware, and I've
been trying to help Firebird work through this issue on IRC, but
unfortunately, don't know much of anything about vala to pinpoint the
issue. I can confirm Shotwell 0.12.3 builds perfectly with Vala 0.17
on 32-bit Slackware without issue, but for some reason, Firebird is
getting this error with the _same_ SlackBuild and deps I'm using (he's
tried vala 0.16.1, 0.17, 0.17.4, etc--all with no success). The only
difference I can see is that he's running 64-bit Slack.