Fix macOS rpath for non-default prefixes

[jonchang]

Installing to a non-/usr/local prefix by setting CMAKE_INSTALL_PREFIX on macOS will result in libhmsbeagle-jni.jnilib failing to load:

% beast -beagle_info
Failed to load BEAGLE library: /opt/homebrew/Cellar/beagle/4.0alpha/lib/libhmsbeagle-jni.jnilib: dlopen(/opt/homebrew/Cellar/beagle/4.0alpha/lib/libhmsbeagle-jni.jnilib, 1): Library not loaded: @rpath/libhmsbeagle.dylib
  Referenced from: /opt/homebrew/Cellar/beagle/4.0alpha/lib/libhmsbeagle-jni.jnilib
  Reason: image not found

This is because the rpath is improperly set to /usr/local:

% otool -l libhmsbeagle-jni.jnilib| grep -i rpath -A2
         name @rpath/libhmsbeagle.dylib (offset 24)
   time stamp 2 Wed Dec 31 16:00:02 1969
      current version 0.0.0
--
          cmd LC_RPATH
      cmdsize 32
         path /usr/local/lib (offset 12)

This patch taken from a recommendation on the cmake wiki. After applying this patch the rpath is correctly set to the install prefix:

% otool -l libhmsbeagle-jni.jnilib | grep -i rpath -A2
         name @rpath/libhmsbeagle.dylib (offset 24)
   time stamp 2 Wed Dec 31 16:00:02 1969
      current version 0.0.0
--
          cmd LC_RPATH
      cmdsize 56
         path /opt/homebrew/Cellar/beagle/4.0alpha/lib (offset 12)

[msuchard]

Thanks, @jonchang !!!