Add -Wno-maybe-uninitialized option to CMAKE_CXX_FLAGS_RELEASE and CMAKE_CXX_FLAGS_RELWITHDEBINFO variables on GCC/Linux.
In Release builds GCC's -Wmaybe-uninitialized warning is unreliable, and thus causes noise that can drown out other warnings. These warnings are now silenced in release mode builds.a
Debug builds seem fine, so flags for debug builds are not touched by this commit.
No functional changes.