Add warning output when using %require, which exists only for compatibility with Bison #1422

Job	Run time
steep-check (head)	26s
check-misc (head)	9s
test-ruby (head, master)	5m 37s
test-memory (head)	36s
test-ruby (3.0, master)	6m 18s
test-windows (head)	2m 22s
ruby-versions / Generate Ruby versions	3s
test-cpp (head, g++)	11s
test (2.5)	27s
test-cpp (head, clang++)	14s
test (2.6)	30s
test (2.7)	29s
test (3.0)	14s
test (3.1)	18s
test (3.2)	18s
test (3.3)	15s
test (head)	16s
	18m 43s

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