Ports are opportunistically linking with libstdcxx

[Ryan Schmidt]

It seems various ports are opportunistically linking with libstdcxx, which fails as follows when the user is trying to build that port universal but libstdcxx is not installed universal:

/opt/local/lib/libstdc++.6.dylib, file was built for unsupported file format

This has been reported against libgdiplus:

https://trac.macports.org/ticket/36438

and perl5.12:

https://trac.macports.org/ticket/36444

Jeremy, can you please take a look? We don't want opportunistic linking.