fix: make configure.ac POSIX-compatible again #397

Job	Run time
macos-latest, cmake, gcc, easy backend	42s
macos-latest, cmake, gcc, gmp backend	47s
macos-latest, cmake, clang, easy backend	38s
macos-latest, cmake, clang, gmp backend	44s
macos-latest, autotools, gcc, easy backend	1m 18s
macos-latest, autotools, gcc, gmp backend	1m 11s
macos-latest, autotools, clang, easy backend	1m 2s
macos-latest, autotools, clang, gmp backend	1m 0s
ubuntu-20.04, cmake, gcc, easy backend	1m 24s
ubuntu-20.04, cmake, gcc, gmp backend	1m 30s
ubuntu-20.04, cmake, clang, easy backend	1m 35s
ubuntu-20.04, cmake, clang, gmp backend	1m 27s
ubuntu-20.04, autotools, gcc, easy backend	2m 8s
ubuntu-20.04, autotools, gcc, gmp backend	2m 1s
ubuntu-20.04, autotools, clang, easy backend	2m 7s
ubuntu-20.04, autotools, clang, gmp backend	2m 0s
	21m 34s

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