Checking telegram-desktop-dev package, Giovanni resolved the glibmm "issue":
https://aur.archlinux.org/cgit/aur.git/tree/PKGBUILD?h=telegram-desktop-dev
I've just merged its changes to my own forkgram private repo and it works. I've pushed my PKGBUILD here:
https://gist.github.com/figue/e53d3edc629ad31e2916e8484465434f
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