py-graph-tool has wrong condition for deciding when to use sparsehash

[ryandesign (Ryan Carsten Schmidt)]

The py-graph-tool Portfile has this code:

    # Clang uses the old libstc++ from gcc 4.2 before OS X 10.9. Boost doesn't
    # include some of the tr1 headers in libstdc++ and defines its own tr1
    # classes. This causes conflicts with sparsehash which insists on using
    # the old tr1 headers.
    if {[string match *gcc* ${configure.compiler}] ||
        ${os.major} >= 13 && ${os.platform} eq "darwin"} {
        depends_lib-append port:sparsehash
    } else {
        configure.args-append --disable-sparsehash
    }

This is out of date. As of MacPorts 2.6.0, libc++ is used by default on 10.6 and up. But you should not infer the C++ standard library based on the OS version. Instead, you should check ${configure.cxx_stdlib} if you need to make a decision based on the C++ standard library.