graphics/epsonscan2: Fix build error on 16-current: "if (a <= b <= c)"

Fix compilation error for "if (a <= b <= c)".

While here:

• Replace PORTVERSION with DISTVERSION.
• Move dependency from boost to BUILD_DEPENDS - the port is not linked with boost libs, but uses includes during build.
• Remove unnecessary "CFLAGS+=".

PR: 294951
Sponsored by: UNIS Labs
Co-authored-by: Vladimir Druzenko <vvd@FreeBSD.org>
MFH: 2026Q2