build: replace `bash`-isms with `sh`-friendly `test` and `case`, cleanup #398

Job	Run time
macos-latest, cmake, gcc, easy backend	46s
macos-latest, cmake, gcc, gmp backend	43s
macos-latest, cmake, clang, easy backend	29s
macos-latest, cmake, clang, gmp backend	29s
macos-latest, autotools, gcc, easy backend	1m 11s
macos-latest, autotools, gcc, gmp backend	1m 34s
macos-latest, autotools, clang, easy backend	1m 6s
macos-latest, autotools, clang, gmp backend	1m 9s
ubuntu-20.04, cmake, gcc, easy backend	1m 33s
ubuntu-20.04, cmake, gcc, gmp backend	1m 30s
ubuntu-20.04, cmake, clang, easy backend	1m 27s
ubuntu-20.04, cmake, clang, gmp backend	1m 25s
ubuntu-20.04, autotools, gcc, easy backend	2m 6s
ubuntu-20.04, autotools, gcc, gmp backend	2m 24s
ubuntu-20.04, autotools, clang, easy backend	2m 11s
ubuntu-20.04, autotools, clang, gmp backend	2m 43s
	22m 46s

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