Blender Git Loki

CMake: Add option to enable -Werror cflag in some areas

It is rather annoying attitude nowadays to use const qualifier all over the
place, including using it for multi-dimensional arrays. This isn't really
supported in GCC prior to version 5.0 because it considers such an arrays
to be a "pointer to a const pointer" which gives implicit casting errors.

It's not possible to disable this particular type of warnings treated as
errors in any GCC version prior to 5.0 as well, meaning currently usage of
-Werror globally in Blender code is not possible at all.

This commit makes it possible to use -Werror in areas which are complaint
with older GCC versions. New advanced CMake options are:

- WITH_COMPOSITOR_WERROR
- WITH_LIBMV_WERROR
- WITH_CYCLES_WERROR