Bmad Distribution - 20230926-0

What's Changed

Allow a separate file for modifications to util/dist_prefs by @jsberg-bnl in #502
Fix write_bmad_lattice_file to properly handle generalized gradient m… by @DavidSagan in #507
Allow gnu_utilities_src to be absent by @jsberg-bnl in #503
Add environment variables for additional include/library search directories by @jsberg-bnl in #504
Add a tao global parameter opti_write_var_file to enable inhibiting writing var*.out files when optimizing by @jsberg-bnl in #508
External deps docs by @ChristopherMayes in #509
Script cleanup by @DavidSagan in #510

Full Changelog: 20230921-0...20230926-0