2.6.0 features

[Joshua Root]

It has now been the customary 2 weeks since the release of MacPorts
2.6.0, so the new features can be freely used in portfiles. In particular:

* New compiler selection options. You can specify required C or C++
standard versions, and/or specify that Fortran, OpenMP, MPI, or TLS is
required. The cxx11 portgroup should no longer be necessary; simply set
'compiler.cxx_standard 2011' instead (and newer standards are supported
just as easily). Some documentation has been written at
<https://trac.macports.org/wiki/CompilerSelection>.

* Ports that need a full Xcode installation to build should set
'use_xcode yes'. This is set automatically if build.type is 'xcode'.
Ports that don't set this option to 'yes' are assumed to be able to be
built with just the Command Line Tools, and MacPorts won't warn about
Xcode being missing.

- Josh