It's correct base behavior. Base sees a port compiled against libstdc++ on a system configured to use libc++, and knows this is most often bad, and needs to be fixed. So it flags this as an error.
If the portfile author investigates and works out that it's OK for this port to do that (as it doesn't use with any other c++ libraries or provide any for other ports to use) then that "error" can be overridden by forcing the configure.cxx_stdlib in the portfile, and base will stop flagging it. K
