Improve libunwind check in configure #5340

Job	Run time
build-openbsd-latest-clang	2m 50s
build-ubuntu-latest-full-featured-clang	1m 30s
build-ubuntu-latest-minimal-clang	1m 22s
build-ubuntu-latest-pcp	39s
build-ubuntu-latest-full-featured-gcc	1m 12s
build-ubuntu-latest-clang-sanitizer	2m 30s
build-freebsd-latest-clang	2m 37s
build-ubuntu-latest-clang-analyzer	1m 35s
lint-whitespace	4s
build-dragonflybsd-latest-gcc	2m 45s
lint-pcp-config	4s
build-unsupported-latest-gcc	36s
build-netbsd-latest-gcc	2m 56s
build-ubuntu-latest-gcc-static	1m 10s
build-ubuntu-latest-minimal-gcc	41s
build-macos-latest-clang	39s
build-solaris-latest-gcc	4m 21s
	27m 31s

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