[xslt] Libxlst doesn't successfully build/install in a flatpak environment?

[Adrian Lucrèce Céleste <adrianlucrececeleste airmail cc>]

On and off (though, mostly off) I've been trying to get libxlst
compiling within flatpak. The only option I set is --prefix=/app, just
in case libxlst doesn't detect this on it's own (though it seems to).
The build process seems to go most of the way through, but then errors
out towards what I assume is the end, the error is as follows:

mv -f .deps/strings.Tpo .deps/strings.Plo
mv -f .deps/saxon.Tpo .deps/saxon.Plo
make[2]: Leaving directory '/run/build/libxlst/libexslt'
make[1]: *** [Makefile:614: all-recursive] Error 1
make[1]: Leaving directory '/run/build/libxlst'
make: *** [Makefile:440: all] Error 2
Error: module libxlst: Child process exited with code 2

A full build log can be found here:
https://gist.github.com/AdrianKoshka/4802b4c99a4bb2430128ba24b94e078c

Really puzzled as to why libxlst doesn't "just work" like most libraries
have for me.