A separate PortIndex for libc++ on older systems

[Joshua Root]

On 2016-8-11 04:09 , Lawrence Velázquez wrote:
> On Aug 10, 2016, at 4:37 AM, Ryan Schmidt <ryandesign at macports.org> wrote:
>
>> We also still need to decide how to differentiate the URLs for libc++ packages from the URLs of the existing libstdc++ packages. One suggestion was to add cxx_stdlib as a variable in archive_sites.conf, and upload the libc++ archives with the same names as the libstdc++ archives but in a new subdirectory, e.g. https://packages.macports.org/libc++/.
>
> One downside of using identical archive names is that we'd be unable to differentiate local libstdc++ archives from libc++ ones based on file name. This distinction could be used to rebuild packages after changing cxx_stdlib, for instance.

There are probably better places to keep metadata than a 255 character 
filename though.

- Josh