Use Meson for the 'reduced' and 'legacy' build variants
From: @rhabacker
There are some differences between Autotools and Meson here:
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Because we detect native atomic operations differently, we can no longer emulate a platform that doesn't have them, such as ARMv4; but modern OSs no longer support ARMv4 and all significant hardware platforms now have native atomic operations, so this is now less of a concern.
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Similarly, we can no longer emulate a platform that doesn't have the getrandom() library function, but that function is available in all relevant glibc versions (such as CentOS >= 8, Debian >= 10, Ubuntu >= 18.04) so testing the fallback path is less of a concern now.
We also no longer try to disable dnotify in the legacy build, but our dnotify code path was removed in 2013, so --disable-dnotify didn't do anything anyway.
[Separated from a larger commit on !378 (merged) —smcv]
Co-authored-by: @smcv