Blender Git Loki

Silence `-Wmaybe-uninitialized` warning on GCC/Linux

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.

Reviewed By: Sergey

Differential Revision: https://developer.blender.org/D8615