Fix `-Wundef` warnings #1212

Job	Run time
ruby-versions / Generate Ruby versions	2s
ubuntu-22.04 2.7	41s
ubuntu-22.04 3.0	18s
ubuntu-22.04 3.1	17s
ubuntu-22.04 3.2	18s
ubuntu-22.04 3.3	22s
ubuntu-22.04 head	18s
ubuntu-20.04 2.7	41s
ubuntu-20.04 3.0	19s
ubuntu-20.04 3.1	19s
ubuntu-20.04 3.2	18s
ubuntu-20.04 3.3	21s
ubuntu-20.04 head	17s
macos-14 2.7	48s
macos-14 3.0	21s
macos-14 3.1	23s
macos-14 3.2	27s
macos-14 3.3	35s
macos-14 head	21s
macos-13 2.7	2m 5s
macos-13 3.0	30s
macos-13 3.1	35s
macos-13 3.2	31s
macos-13 3.3	40s
macos-13 head	39s
windows-latest 2.7	3m 55s
windows-latest 3.0	2m 30s
windows-latest 3.1	2m 17s
windows-latest 3.2	3m 24s
windows-latest 3.3	3m 22s
windows-latest ucrt	2m 1s
windows-latest mswin	2m 20s
ubuntu-20.04 jruby	1m 25s
ubuntu-20.04 jruby-head	1m 23s
	35m 3s

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