buildsys: add `-Wall` and `-Wextra`, remove duplicate flags, set stan… #91

Job	Run time
call-build-freebsd / build-freebsd	3m 23s
call-build-alpine / build-alpine	37s
call-build-fedora / build-fedora	48s
call-build-macos / build-macos	1m 31s
call-build-solaris / build-solaris	6m 9s
call-build-ubuntu / build-ubuntu	53s
	13m 21s

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