Log Message:
vte3: un-break builds with default GCC < 10

As of 0.68, this package expects C++20, and also fails if GCC < 10 or
clang < 11. (I don't see an equivalent to GCC_REQD for clang.) This
isn't necessarily a satisfactory fix, but at least gets it to build
again, un-breaking DE meta-pkgs like xfce4, mate, and gnome. It needs
further investigation.

The commit that enforces this new (high) bar in meson.build is:
https://github.com/GNOME/vte/commit/eec33cce0b9359fddda35703e22bffd1f8f28852
I haven't had time to examine the code to see what C++20 features are
involved.