g++-4.2 and -stdlib on 10.6

[Mojca Miklavec]

Hi,

I naively added -stdlib=${configure.cxx_stdlib} to a port, but this
broke the compilation on 10.6:

cc1plus: error: unrecognized command line option "-stdlib=libstdc++"

(See https://trac.macports.org/ticket/44145.)

What's the best/proper way to handle this setup, so that the Portfile
would support building the package on 10.6 both with libstdc++ as well
as with libc++? (Blacklisting the default compiler is probably one
option, but not necessarily the best one.)

Mojca