Fix macOS compatibility issues and update Homebrew paths

[coloboxp]

This commit addresses several compatibility issues to ensure the codebase works well on modern macOS (M4):

1. Replace deprecated BSD function calls:

   • Replace rindex() with strrchr() for POSIX compliance
   • This change is reflected in dpymain.c and potentially other files

2. Update X11 paths for macOS Homebrew users:

   • Change X11 include path to /opt/homebrew/include
   • Change X11 library path to /opt/homebrew/lib

3. Reorganize header includes:

   • Properly order system headers before project headers
   • Ensure proper compilation on platforms with strict include requirements

These changes maintain backward compatibility while ensuring the codebase compiles and runs correctly on modern systems.