bsd.port.mk: Remove confusing double "/" from _DISTDIR _DISTDIR is a variable which stores the distfiles directory of the port that is currently being worked on. In practice, it is usually just the DISTDIR, although some ports also define a subdirectory. Currently, a slash separating DISTDIR from DIST_SUBDIR is added to _DISTDIR no matter if DIST_SUBDIR is set or not. This may cause confusion to ports users by looking like a bug while it is not. In particular, this patch casuses diagnostic logs of failed archive extraction to look like this: ===> Failed to extract "/portdistfiles/ttf-iosevka-ss05-4.1.1.zip". instead of this: ===> Failed to extract "/portdistfiles//ttf-iosevka-ss05-4.1.1.zip".