To compile and link on Mac OS X 10.6, make the following changes to Makefile:
   1) Replace MacOSX10.5.sdk with MacOSX10.6.sdk
   2) Replace /usr/lib/gcc/powerpc-apple-darwin9 with
          /usr/lib/gcc/powerpc-apple-darwin10

To compile and link on Mac OS X Big Sur
   1) Make sure that the Xcode is updated with v13 or later, depending on the
      OS version. The Xcode should be placed at /Applications/Xcode.app folder.
   2) For gfortran, make sure its version is also updated.
      https://github.com/fxcoudert/gfortran-for-macOS/releases/tag/11.2-bigsur-intel
      If you encounter this error:
       gfortran: error: unrecognized command line option ‘-fallow-argument-mismatch’
      it means the gfortran might be too old. Either upgrade the gfortran, or
      edit the Makefile. Search for the line with ‘-fallow-argument-mismatch’
      option and remove this option from the line.

