CI: enable gcc-15 on macOS

[perazz]

Enable GCC 15 support in CI with necessary fixes for compiler compatibility issues.

• CI Changes: Add GCC 15 to CI matrix (macOS only due to availability constraints)
• GCC 15 Bug Fixes:
  • Replace array constructors with explicit loops in add_strings utilities to work around gfortran-15 stack overflow issues
  • Move large stack variables to heap allocation to prevent stack overflow crashes
  • Fix C++ interface compatibility with proper c_int types
• Compiler Detection: Add GNU vs Clang C++ backend detection for proper linking on macOS