This is a bit unusual, but legitimate way to define RUN_DEPENDS and probably just requires a regexp to be expanded a little? This is for lang/gcc and I don't know how to nicely tweak that to avoid this waring. (In a way it may be the bigger sibling of bug #202052 from a while ago.)
Fixed in 2.19.9.
This update generates an incorrect warning for all ports that have python depends and flavors. The g removes ${PY_FLAVOR} that is checked later.