Warn when XDG_RUNTIME_DIR isn't set, as per XDG Base Dir Spec

http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html
says: "If $XDG_RUNTIME_DIR is not set applications should fall back to a
replacement directory with similar capabilities and print a warning message."

Better warn that not all the guarantees of XDG_RUNTIME_DIR are available,
and push unixes who haven't done so yet, to set things up correctly.

Change-Id: Ie3998e8c1d14e4fdf8d58dfb2932f0ad708fba50
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/io/qstandardpaths_unix.cpp

@@ -127,6 +127,7 @@ QString QStandardPaths::writableLocation(StandardLocation type)
                     return QString();
                 }
             }
+            qWarning("QStandardPaths: XDG_RUNTIME_DIR not set, defaulting to '%s'", qPrintable(xdgRuntimeDir));
         }
         // "The directory MUST be owned by the user"
         QFileInfo fileInfo(xdgRuntimeDir);