Hi,

Gnash is incompatible with Boost 1.80.0. The latest commit in Gnash, 
from 2019, says:

-This is the README file for the Gnash-0.8.8 release (or snapshot).
+Gnash is currently not being actively maintained, as most of the internet
+has moved to using HTML5. There are literally millions of flash files
+that Gnash can play, as well as YouTube videos. As Adobe is letting their
+flash player die on most platforms, Gnash will be critical in the future
+for anyone wanting to play flash files for historical reasons. Gnash's code
+is very portable, and should be compilable for many years, long after
+Adobe flash is dead.

It may not be feasible to support Gnash long-term. You could use 
Lightspark instead, which is still actively maintained and up to date in 
Guix, or use time-machine to move to a commit where Gnash was building, 
prior to the core-updates merge that introduced Boost 1.80.0.

IMO we should remove Gnash from the repository, unless someone wants to 
patch it for the latest Boost.

Vinicius