Build libpng manually to create fat macOS binaries #940

Job	Run time
macos-static	27s
unix (ubuntu-20.04, g++, make)	1m 47s
windows-mingw-build (32)	53s
windows (32, windows-2019)	1m 47s
unix (ubuntu-20.04, g++, cmake)	1m 46s
windows-mingw-build (64)	53s
windows (32, windows-2022)	1m 37s
unix (ubuntu-20.04, clang++, make)	1m 48s
windows (64, windows-2019)	1m 37s
unix (ubuntu-20.04, clang++, cmake)	1m 48s
windows (64, windows-2022)	1m 39s
unix (ubuntu-22.04, g++, make)	1m 47s
unix (ubuntu-22.04, g++, cmake)	1m 46s
unix (ubuntu-22.04, clang++, make)	1m 47s
unix (ubuntu-22.04, clang++, cmake)	1m 47s
unix (macos-14, clang++, make)	1m 49s
unix (macos-14, clang++, cmake)	1m 47s
windows-mingw-testing (windows-2019, 32)	37s
windows-mingw-testing (windows-2019, 64)	37s
windows-mingw-testing (windows-2022, 32)	37s
windows-mingw-testing (windows-2022, 64)	40s
	29m 16s

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