Switching to Libc++ on older systems by default

Wiki page for the setup: Libc++ on older systems

Tickets to be addressed:

• #50448

  Change filenames of binary packages built against libc++ on < 10.9

  #53634

  clang++-mp-4.0 -stdlib=macports-libstdc++ should be in C++11 mode by default

  #55471

  Use separate portindexes for libc++ on older systems

See also:

• #49903

  libcxx: undo changes to system directories when deactivating

  #52000

  Add xz to "base" to allow changing the default compression format to .tar.xz in the future

cxx_stdlib might need to be one additional criteria to respect when selecting an archive site here:

• ​https://github.com/macports/macports-ports/blob/master/_resources/port1.0/fetch/archive_sites.tcl
• ​https://github.com/macports/macports-base/blob/995dde8476c48580db4f6eedfde09e90dc5e8c99/src/package1.0/portarchivefetch.tcl#L66

Then the archive site would no longer be used if anyone changes the default in macports.conf.