build: use non-deprecated compiler flag when building jruby extension #639

Job	Run time
ruby-versions / Generate Ruby versions	5s
ubuntu-22.04 2.7	49s
ubuntu-24.04 3.2	19s
ubuntu-24.04 3.0	14s
ubuntu-24.04 3.4	19s
ubuntu-24.04 jruby-head	1m 12s
ubuntu-24.04 3.3	26s
ubuntu-22.04 3.2	19s
ubuntu-24.04 head	22s
ubuntu-22.04 3.1	15s
ubuntu-24.04 4.0	22s
ubuntu-24.04 2.7	47s
ubuntu-22.04 4.0	20s
ubuntu-24.04 truffleruby-head	50s
ubuntu-22.04 jruby-head	1m 26s
ubuntu-22.04 3.4	17s
ubuntu-24.04 debug	29s
ubuntu-22.04 truffleruby-head	49s
ubuntu-22.04 debug	30s
ubuntu-24.04 3.1	19s
ubuntu-22.04 3.3	17s
ubuntu-22.04 3.0	14s
ubuntu-22.04 head	24s
	11m 24s

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