SCons: Don't enable any `-Werror=return-type` outside `werror=yes`
SCons: Don't enable any `-Werror=return-type` outside `werror=yes`

We shouldn't presume that future compilers will not have false positives or
find new occurrences of this warning, which would break compiling old versions
of the engine without passing custom `CXXFLAGS`.

Follow-up to #60652.

(cherry picked from commit e55d30768a83f027f2c36e106e0ad5320723709d)

ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot