Failed to build gem native extension with GCC15

[Madeorsk]

I can't build the gem native extension using GCC15. It seems to be related to this issue: https://bugs.ruby-lang.org/issues/21290

-std=c99 flag seems to be the problem. Using append_cflags fixes the issue in another gem: https://github.com/RubyCrypto/ed25519/pull/45/files#diff-805cf1ea9824de19b94dce429b3a3544fd9cb0b3cc278e571776c7d672cc1dfa