github actions: Add a GNU autotools build.
This only runs on linux and macOS, but supports the important `distcheck` target. We ask homebrew to install libtool and pkg-config on macOS. These are required for the autotools build, but aclocal at least seems not to be available in the latest image. We don't do a general homebrew update since that can take several minutes, at least doubling the time it takes to runthhe job. Just using the defaults the image provides is a better tradeoff.
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