gitlab CI: compare the meson and autotool builds after install

Peter Hutterer requested to merge whot/xorgproto:wip/ci-compare-meson into master

Sits on top of !29 (merged)

Build and install with meson, build and install with autotools and then run diff to compare the two directory trees. They should be the same.

This does not install the legacy protocols, they're behind a configure switch. The spec-build is disabled in autotools because we know meson doesn't do that yet, so no point in comparing those.

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