fix gcc 5+ compilation -Werror=unused-variable

@jsola can you please compile on your laptop (clang) ?

On the way.

But what's this UNUSED thing? What's the intention of this? What's the effect of this? How do we make use of this? What's the scope of this?

OK I compile correctly

It's a small macro that tells the compiler that the variable is not used hence it should throw a -Werror=unused-variable about it.

It seems that more recent versions of gcc (here 5.4) perceive the const bool FooRegistered used in factory registration has not being used - it actually is not.

I see

well I compile, no prb. I'll merge to master when you want.

good to go

merged

mentioned in commit e452aef3