graphics/bmeps: Update 4.37.1 => 4.39.0

Changelog:
https://sourceforge.net/p/dktools/wiki/News%20and%20Change%20Log/#h-4390-2025-08-22

• Replace PORTVERSION with DISTVERSION in MASTER_SITES and DISTNAME.
• Replace libpng16.so with libpng.so in LIB_DEPENDS.

PR: 289805