Make library include guards more consistent #489

Job	Run time
test-linux-add_subdirectory	38s
test-linux-fetchcontent	54s
test-linux-findpackage	26s
test-macos-fetchcontent	38s
test-macos-findpackage	24s
test-macos-add_subdirectory	33s
test-mingw-add_subdirectory	2m 52s
test-mingw-fetchcontent	2m 57s
test-mingw-findpackage	2m 15s
test-windows-add_subdirectory	1m 57s
test-windows-fetchcontent	1m 9s
test-windows-findpackage	1m 53s
	16m 36s

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