This version is not supported by viking, leading to an intentional compilation error. Looking at the documentation this change was probably introduced in gpsd 3.12, but I have not explicitly verified this.
For reference see the Gentoo downstream bug report: https://bugs.gentoo.org/show_bug.cgi?id=541646
I've reviewed the GPSD API changes and they only relate to AIS stuff which Viking doesn't use.
Thus it seems safe to enable use of GPSD_API_MAJOR_VERSION == 6 as equivalent to v5 within Viking.
I've updated GPSD on my system, rebuilt against the latest version and Viking is working OK.
Thanks for reporting this issue.
Included in the Viking 1.6 release.