Adding support for building using GCC 15

[memoarfaa]

Summary

Closes #444
Fixes #444
This pull request addresses build failures in the efi branch of grub4dos when using GCC 15 or newer, as reported in #444.
It ensures compatibility with newer GCC versions by adjusting the configure script to add -std=gnu17 to the CFLAGS if GCC 15+ is detected, avoiding build errors with incompatible pointer types and function arguments.

What does this PR do?

• Updates configure to detect GCC 15 and newer.
• Adds -std=gnu17 to CFLAGS for compatibility with the new GCC default (-std=gnu23).
• Resolves build errors related to incompatible pointer types and too many arguments to functions.

Testing

• Tested on Artix GNU/Linux (rolling release) with GCC 15.1.1
• Tested on Ubuntu 24.04.2 LTS WSL with GCC 15.1.0
• Tested on Ubuntu 25.10 Hyper-V Manager 15.2.0
• Ensured that the build passes without the previously encountered errors.

Additional Notes

• Please let me know if further changes or testing on different environments are required.

[dreirund]

Why is gcc15="14.9.0"?