Copy the $PATH, $MANPATH, and LC_ALL variables from the interactive login shell to Emacs. This is necessary on macOS, as its GUI starts programs within a minimal environment.

In practice, this fixes various problems, such as the *compilation* buffer not finding emacs, fortune not finding strfile, geiser-mode not finding guile, emacs not finding gcc for native compilation, as well as, spurious locale warnings.

(dolist (line (process-lines "bash" "-l" "-c" "export"))
  (when (string-match "^.* \\(.*?\\)=\\\"\\(.*\\)\\\"$" line)
    (let ((name (match-string 1 line))
          (value (match-string 2 line)))
      (setenv name value)
      (when (string= name "PATH")
        (setopt exec-path (split-string value path-separator))))))

(There is package on MELPA for this, but it is unnecessarily complex, and we need to run this before the package system is initialized.)

[I published this code for the wider Emacs community at StackOverflow.]