ispow2: Better fallback implementation #758

Job	Run time
build-cross (x86_64-w64-mingw32)	1m 52s
build-windows (x86)	1m 6s
build-cross (i686-w64-mingw32)	1m 22s
build-windows (x64)	1m 39s
build (ubuntu-22.04, gcc, autotools)	50s
build (ubuntu-22.04, gcc, cmake)	43s
build (ubuntu-22.04, clang, autotools)	52s
build (ubuntu-22.04, clang, distcheck)	52s
build (ubuntu-22.04, clang, cmake)	56s
build (macos-13, gcc, autotools)	1m 16s
build (macos-13, gcc, cmake)	1m 14s
build (macos-13, clang, autotools)	1m 13s
build (macos-13, clang, distcheck)	1m 55s
build (macos-13, clang, cmake)	1m 37s
build (macos-14, gcc, autotools)	54s
build (macos-14, gcc, cmake)	49s
build (macos-14, clang, autotools)	53s
build (macos-14, clang, distcheck)	1m 2s
build (macos-14, clang, cmake)	48s
release	0s
	21m 53s

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