mail/mu: Update 1.12.13 => 1.12.15
Changelog:
https://github.com/djcb/mu/releases/tag/v1.12.14
https://github.com/djcb/mu/releases/tag/v1.12.15
Improve port:
- Fix build with emacs installed.
- Fix warning from portclippy.
PR: 293277 292576
MFH: 2026Q1