Blender Git Loki

Fix/workaround GCC bug about -Wno-implicit-fallthrough

For some reason GCC-6 successfully compiles test program with
-Wno-implicit-fallthrough passed via command line. It just
silently ignores the unknown arguments which are starting with
-Wno-.

The issue is, if some other waning happens in the code, then
GCC will complain about unknown -Wno- argument which is not
supported by current GCC version.

This makes some misleading warning prints about unknown
command line argument when any other warning happens in code
from extern/.