Fix build with Apple Silicon and modern toolchain

[jlcuadros]

This fixes building ChessX natively on Apple Silicon Macs.

Tested on:

• macOS 26.5.1
• Apple Silicon arm64
• AppleClang 21
• CMake 4.4
• Qt 5.15

Changes:

• Update C++ standard from C++11 to C++17 because the code uses std::as_const.
• Add src/gui to quazip include directories so qt6compat.h can be found.

Result:
ChessX builds and runs as a native arm64 macOS application.