XProxy migrating to C++14

[Georg Rudoy]

Hi all,

The XProxy plugin is migrating to C++14. Thus, it is now only
buildable with -DUSE_CPP14=True flag passed to CMake.

This particular plugin should be buildable with both gcc 4.9+ and
clang 3.4+, but other C++14 plugins (like Azoth Sarin or Snails) are
only buildable with clang 3.4+ (and probably gcc 5, we'll see when it
gets released). Take care of that.

--
Georg Rudoy