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mkrelease: fix three regressions, add simple 'dist' CI job #92

Job	Run time
Cygwin	1m 55s
macOS	19s
Linux (clang sanitizers, clang, test, clang, -fsanitize=address,undefined,signed-integer-overflow...	29s
Linux (default, valgrind, test-memory)	3m 2s
Windows	52s
	6m 37s

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