Fixed compatibility issues under Xcode 15.0

[Magic-Xin]

Fixed compatibility issues under Xcode 15.0 which seems caused by C++17 standard.

[FunJim]

Could someone review this PR? Currently pyopenjtalk build is failed on macOS Sonoma (see r9y9/pyopenjtalk#75)

[r9y9]

I am sorry that I was unable to check this PR. I will try to make some time in this weekend to look into this issue

[w568w]

Bump. @r9y9, do you have time to review this now?

This is a simple PR that removes the deprecated and now invalid register keywords from src/mecab.

It seems the original author intended to improve performance with register, but compilers have ignored this keyword for a long time. Removing it won't affect the final binary, thus this PR is purely a clean-up, eliminating an outdated and ineffective part of the code!

Rationale: Clang 18.1.8 has already removed support for register and treats it as an error. GCC 14.1.1 is slightly more tolerant but will also stop supporting it very soon.

[r9y9]

I apologize for the super delay. I checked this PR works OK with clang 18

clang version 18.1.8 (https://github.com/llvm/llvm-project.git 3b5b5c1ec4a3095ab096dd780e84d7ab81f3d7ff)
Target: arm64-apple-darwin22.6.0
Thread model: posix

[r9y9]

Thank you very much, @Magic-Xin and @w568w