build: allow incremental rebuilds #453

Job	Run time
build (ubuntu-latest)	2m 27s
build (ubuntu-latest, --disable-curses)	2m 28s
build (ubuntu-latest, --disable-lua)	2m 19s
build (ubuntu-latest, --disable-tre)	3m 10s
build (ubuntu-latest, --disable-selinux)	2m 47s
build (ubuntu-latest, --disable-acl)	2m 31s
build (ubuntu-latest, --disable-help)	2m 46s
build (ubuntu-20.04)	2m 43s
build (ubuntu-20.04, --disable-curses)	2m 37s
build (ubuntu-20.04, --disable-lua)	2m 21s
build (ubuntu-20.04, --disable-tre)	2m 33s
build (ubuntu-20.04, --disable-selinux)	2m 44s
build (ubuntu-20.04, --disable-acl)	2m 43s
build (ubuntu-20.04, --disable-help)	2m 43s
	36m 52s

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