bash: configure default value of BASH_LOADABLES_PATH

See: https://www.gnu.org/software/bash/manual/html_node/Bash-Builtins.html#index-enable
See: http://git.savannah.gnu.org/cgit/bash.git/tree/config-top.h#n80

This enables shorter syntax for loading stock Bash builtins. For example,
instead of doing this:

    enable -f $(brew --prefix)/lib/bash/csv csv

the user can now load the `csv` builtin directly:

    enable csv

To complete the example, we can now do this:

    $ echo $BASH_LOADABLES_PATH
    /opt/homebrew/Cellar/bash/5.2.32/lib/bash:/usr/local/lib/bash:/usr/lib/bash:/opt/local/lib/bash:/usr/pkg/lib/bash:/opt/pkg/lib/bash:.
    $ type csv
    -bash: type: csv: not found
    $ enable csv
    $ type csv
    csv is a shell builtin
    $ help csv
    csv: csv [-a ARRAY] string
       Read comma-separated fields from a string.

       Parse STRING, a line of comma-separated values, into individual fields,
       and store them into the indexed array ARRAYNAME starting at index 0.
       If ARRAYNAME is not supplied, "CSV" is the default array name.
    $ csv 'a,"b",c'
    $ declare -p CSV
    declare -a CSV=([0]="a" [1]="b" [2]="c")
    $ enable -n csv
    $ type csv
    -bash: type: csv: not found

Formula/b/bash.rb

@@ -112,11 +112,24 @@ def install
  # Homebrew's bash instead of /bin/bash.
  ENV.append_to_cflags "-DSSH_SOURCE_BASHRC"
 
+ bash_loadables_path=[
+ "#{lib}/bash",
+ # Stock Bash paths; keep them for backwards compatibility.
+ "/usr/local/lib/bash",
+ "/usr/lib/bash",
+ "/opt/local/lib/bash",
+ "/usr/pkg/lib/bash",
+ "/opt/pkg/lib/bash",
+ ".",
+ ].join(":")
+ ENV.append_to_cflags "-DDEFAULT_LOADABLE_BUILTINS_PATH='\"#{bash_loadables_path}\"'"
+
  system "./configure", "--prefix=#{prefix}"
  system "make", "install"
  end
 
  test do
- assert_equal "hello", shell_output("#{bin}/bash -c \"echo -n hello\"")
+ assert_equal "hello", shell_output("#{bin}/bash -c 'echo -n hello'")
+ assert_equal "csv is a shell builtin\n", shell_output("#{bin}/bash -c 'enable csv; type csv'")
  end
 end