lang/smlnj: Update to 110.99.6.1

Add python:env and shebangfix to USES.
Use target-OPTION-{on,off} instead of if-statements.
Add targets-recompile.customized file instead of echo commands.
Switch to DISTVERSION.
Pet portfmt.

Changelogs:
https://www.smlnj.org/dist/working/110.99.6.1/110.99.6.1-README.html
https://www.smlnj.org/dist/working/110.99.6/110.99.6-README.html
https://www.smlnj.org/dist/working/2024.1/2024.1-README.html
https://www.smlnj.org/dist/working/110.99.5/110.99.5-README.html
https://www.smlnj.org/dist/working/2023.1/2023.1-README.html
https://www.smlnj.org/dist/working/110.99.4/110.99.4-README.html
https://www.smlnj.org/dist/working/2022.1/2022.1-README.html
https://www.smlnj.org/dist/working/110.99.3/110.99.3-README.html
https://www.smlnj.org/dist/working/2021.1/2021.1-README.html
https://www.smlnj.org/dist/working/110.99.2/110.99.2-README.html
https://www.smlnj.org/dist/working/110.99.1/110.99.1-README.html
https://www.smlnj.org/dist/working/110.99/110.99-README.html

PR: 283301
Reported by: Alexey Vyskubov <alexey@ocaml.nl>
Approved by: submitter is maintainer