Fix fuzz warnings and deny warnings by default in build system #2770

Job	Run time
duplicates	3s
spell_check	7s
build_system	26s
build (gcc-15.deb, --test-libcore)	1m 9s
build (gcc-15.deb, --std-tests)	1m 8s
build (gcc-15.deb, --extended-rand-tests)	1m 7s
build (gcc-15.deb, --extended-regex-example-tests)	1m 11s
build (gcc-15.deb, --test-successful-rustc --nb-parts 2 --current-part 0)	1m 7s
build (gcc-15-without-int128.deb, --std-tests)	1m 49s
build (gcc-15.deb, --extended-regex-tests)	1m 11s
build (gcc-15-without-int128.deb, --extended-regex-example-tests)	1m 5s
build (gcc-15.deb, --test-successful-rustc --nb-parts 2 --current-part 1)	2m 12s
build (gcc-15.deb, --projects)	1m 13s
build (gcc-15-without-int128.deb, --test-libcore)	2m 8s
build (gcc-15-without-int128.deb, --test-successful-rustc --nb-parts 2 --current-part 0)	1m 17s
build (gcc-15-without-int128.deb, --extended-regex-tests)	1m 9s
build (gcc-15-without-int128.deb, --extended-rand-tests)	1m 7s
build (gcc-15-without-int128.deb, --projects)	1m 14s
build (gcc-15-without-int128.deb, --test-successful-rustc --nb-parts 2 --current-part 1)	2m 23s
success	2s
	23m 8s

Provide feedback