Older compilers, e.g., GCC 4.4., do not understand the
"simple-type-specifier" syntax of C++11's "friend" declarations.
Typical responses are
error: a class-key must be used when declaring a friend
error: friend declaration does not name a class or function
Restore build on older compilers (e.g., GCC prior to 4.7) by
inserting the 'class' keyword.