If the user forgets the final `()` to invoke the function, they will now
receive a warning. This is only relevant for void-returning functions
where the result is not otherwise stored. For functions which return a
value, this is both impossible to check and unnecessary, because they
will probably receive other compile errors down the line.

Even though we used feature-test macros, Clang emitted warnings about
using features from C++Future. This change silences those warnings by
using _Pragma to ignonre the particular warning flag for only the
offending constructs.

This requires quite a bit of compiler detection preprocessor code, but
short of Clang getting its act together with these warnings around
feature-test macros, there's not much else we can realistically do.