Fix native extension build with GCC 15

[davue]

OS: Fedora 42
Ruby: 3.2.1
GCC: 15.1.1

The native extension build is broken with GCC 15 resulting in the same errors as described here: https://bugs.ruby-lang.org/issues/21290

CFLAGS should be appended with append_cflags instead of directly appending them which fixes the problem.

[sudoremo]

Any chance this will get merged in the coming weeks? Many thanks.

[davue]

Duplicate of #229

[davue]

I cannot reopen the older PR because it is not mine so I'm tracking this here.

I still think we should merge this for two reasons:

• It is generally recommended to use append_cflags instead of manually appending it (or is there a reason it was done like this here?)
• To support Ruby versions < 3.4.5

[composerinteralia]

Using append_cflags makes sense to me and matches what the docs recommend: https://docs.ruby-lang.org/en/master/extension_rdoc.html#label-Prepare+extconf.rb