Re-use built object files across targets #1981

Job	Run time
freebsd14 (CMake) (LLVM)	7m 6s
macos-14 (Autotools) (LLVM)	5m 37s
debian:unstable (CMake) (GNU)	3m 22s
macos-14 (CMake) (LLVM)	9m 55s
debian:12 (Latest Mapnik) (CMake) (GNU)	3m 41s
fedora:rawhide (CMake) (GNU)	3m 28s
ubuntu:22.04 (Latest Mapnik) (CMake) (GNU)	3m 29s
ubuntu:devel (CMake) (GNU)	3m 12s
ubuntu:22.04 (Autotools) (GNU)	2m 19s
ubuntu:22.04 (Autotools) (LLVM)	2m 3s
ubuntu:22.04 (CMake) (GNU)	3m 7s
ubuntu:22.04 (CMake) (LLVM)	3m 4s
	50m 23s

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