Ox: Attribute Grammar Compiling System oxml2m4

- [regression testing]: the macOS Command Line Tools for Xcode 26.3 clang 'integrated assembler' warns about overriding deployment version from '16.0' to '26.0' invoked from these compilers
- special case handling for Homebrew LLVM C/C++ compiler versions 16-20: add flag '-mmacosx-version-min=<macOS major version>.0' to 'CFLAGS/CXXFLAGS'
- [regression testing]: avoid warnings from the macOS Command Line Tools for Xcode 26.3 clang when linking with libraries built for macOS version 26.0