• Update `graphics/alembic' to version 1.7.4 [*]
• Utilize PLIST_FILES instead of pkg-plist to avoid passing ${PORTVERSION} and ${PORTVERSION:R} via PLIST_SUB (not very elegant, but quite clear)

Found out via: repology.org [*]

Because GitHub releases (tarballs) are not fetched with correct modification
time, set TIMESTAMP to 1507854327 which corresponds to commit aefa609 tagged
as this release.