[Bug c/66918] Disable "inline function declared but never defined" warning

[eugene.zelenko at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66918

--- Comment #4 from Eugene Zelenko <eugene.zelenko at gmail dot com> ---
(In reply to Manuel López-Ibáñez from comment #3)
> Does Clang have an option for this? GCC could use the same name.
> 
> (The same warning exists in the C++ FE, thus it should be controlled by the
> same option).

I tried small example with Clang 3.7 https://gcc.godbolt.org and it looks like
it doesn't have such warning (I used -Weverything -std=C++11 and C++14).

GCC already has -Wunused-function and it seems reasonable to extend it to
inline functions.