It's news to me that MacPorts gcc has /opt/local/include in its default search path, and my first response is that I would not want it to do that, especially if as you say MacPorts clang doesn't do the same thing. So I would consider this a bug in MacPorts gcc that I would want to fix. Does anyone know how to fix it?
Yes, that seems like the reason why MacPorts gcc is able to find MacPorts openssl and clang is not. The usual way to get a compiler to find MacPorts software is to supply -isystem/opt/local/include in CPPFLAGS and -L/opt/local/lib in LDFLAGS. Different build systems may have different requirements. I'm not familiar with how pip does it.
