Make macports caffe compile with c++11 or the modern compiler

I'm trying to compile caffe on my Mac (11.5.2) with MacPorts. I've gotten nearly everything to compile (a lot of work, because a lot of the MacPorts files have inconsistencies in them), but I can't get caffe to compile because it's using protobuf instead of protobuf3, and protobuf requires C++11 or above, and the portsfile doesn't specify this.

So it seems that the easiest way to move forward is to specify a CXXFLAGS of -std=c++11 or specifying to use a version of clang that defaults to C++11 or above, but the documentation for MacPorts is extensive and not very user-friendly, so I have no idea where to start to accomplish this relatively simple goal.

How do I specify a CXXFLAGS="--std=c++11" or specify a clang that defaults to C++11 or above when building MacPorts ports?