GCC 5 is able to prove that in some translation units the static Logger
instance is defined but never used, and generates an unused variable
warning for each one of them. These warnings become fatal errors when
building in debug mode, due to -Werror.
Silence the compiler by not declaring the unused variables.
refs #2767
Change-Id: Icacc579d917d660582221ef090954ec827f46149
Davide Pesavento
Spyridon Mastorakis