README: improve text about macOS and `dsymutil` #948

Job	Run time
test-linux-add_subdirectory (On)	26s
test-linux-fetchcontent (On)	36s
test-linux-add_subdirectory (Off)	1m 1s
test-linux-fetchcontent (Off)	31s
test-linux-findpackage (On)	19s
test-linux-findpackage (Off)	20s
test-macos-add_subdirectory (On)	24s
test-macos-fetchcontent (On)	29s
test-macos-add_subdirectory (Off)	26s
test-macos-fetchcontent (Off)	25s
test-macos-findpackage (On)	20s
test-macos-findpackage (Off)	22s
test-mingw-add_subdirectory (On)	1m 44s
test-mingw-fetchcontent (On)	1m 42s
test-mingw-add_subdirectory (Off)	1m 46s
test-mingw-findpackage (On)	1m 16s
test-mingw-fetchcontent (Off)	1m 52s
test-windows-add_subdirectory (On)	55s
test-mingw-findpackage (Off)	1m 28s
test-windows-fetchcontent (On)	1m 42s
test-windows-findpackage (On)	1m 34s
test-windows-add_subdirectory (Off)	58s
test-windows-fetchcontent (Off)	1m 41s
test-windows-findpackage (Off)	1m 13s
	23m 30s

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