tests

Use CFLAGS if set as-is, match CXXFLAGS behavior #1272

Job	Run time
test_cygwin (39, windows-latest)	3m 39s
test_msys2_mingw (mingw64, x86_64, gcc, g++)	1m 41s
collateral (diffcov)	1m 0s
test (3.9, ubuntu-latest)	59s
test_msys2_mingw (mingw32, i686, gcc, g++)	1m 47s
collateral (docs)	25s
test_msys2_mingw (ucrt64, ucrt-x86_64, gcc, g++)	1m 38s
test (3.9, macos-latest)	1m 28s
ci_setuptools (3.10, ubuntu-latest)	3m 50s
test_msys2_mingw (clang64, clang-x86_64, clang, clang++)	1m 47s
test (3.9, windows-latest)	2m 7s
test_msvc_python_mingw	2m 16s
test (3.13, ubuntu-latest)	56s
test (3.13, macos-latest)	49s
test (3.13, windows-latest)	3m 6s
test (3.10, ubuntu-latest)	46s
test (3.11, ubuntu-latest)	41s
test (3.12, ubuntu-latest)	48s
test (3.14, ubuntu-latest)	1m 1s
test (pypy3.10, ubuntu-latest)	1m 49s
check	2s
release	0s
	32m 35s

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