Replace USE_GCC=yes by a strict build dependency on lang/gcc since
only that guarantees that a binary (or actually symlink) named gcc
is present which this ports apparently requires.

USE_GCC=yes allows users to adjust the default version of GCC via
Mk/bsd.defaults-versions.mk, and with the change coming via PR218330
lang/gcc5 is going to be used instead of the (nearly identical, for
now) lang/gcc shortly.

PR: 218475, 218330
Approved by: cem (maintainer)