MFH: r484946

multimedia/gnome-mpv: chase compiler used by mpv

On GCC architectures OPENCV option in multimedia/ffmpeg appears to
require consumers to pass -L${_GCC_RUNTIME}. So, use modern GCC where
this is implicit.

usr/local/lib/libopencv_core.so.3.4: undefined reference to `std::detail::_List_node_base::_M_hook(std::detail::_List_node_base*)@GLIBCXX_3.4.15'
usr/local/lib/libopencv_core.so.3.4: undefined reference to `std::__throw_bad_function_call()@GLIBCXX_3.4.14'

PR: 233120
Submitted by: linimon
Approved by: ports-secteam blanket