gnu99 check in extconf seems to prevent Makefile generation on JRuby, MacOS 15

A check for the flag -std=gnu99 appears to fail and prevent generating the Makefile on JRuby. Other gems with "plain" C libraries seem to install correctly on this system (JRuby, Java 21, MacOS 15):

$ bundle exec rake compile
Source locally installed gems is ignoring #<Bundler::StubSpecification name=resolv version=0.6.0 platform=ruby> because it is missing extensions
🚨🚨🚨🚨 Skipping RubyMemcheck: #<LoadError: no such file to load -- ruby_memcheck> 🚨🚨🚨🚨
mkdir -p tmp/java/rbs_extension/3.4.2
cd tmp/java/rbs_extension/3.4.2
/Users/headius/work/jruby/bin/jruby -I. ../../../../ext/rbs_extension/extconf.rb
checking for whether -std=gnu99 is accepted as CFLAGS... *** ../../../../ext/rbs_extension/extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.

I have not investigated this yet myself.

Note that in order to bundle, some gems need to be masked out as described in #2432.

Start with ext/rbs_extension/extconf.rb and the mkmf.log produced by the shown JRuby command. Reproduce the failed Makefile generation on JRuby with Java 21 and macOS 15, then confirm that the extension can generate its Makefile and still builds in the affected environment.