Without the change -fno-common toolchain like llvm-11 fails as:
ld: libcamlrun.a(minor_gc.o):/build/ocaml/byterun/caml/major_gc.h:67: multiple definition of
`caml_major_ring'; libcamlrun.a(stacks.o):/build/ocaml/byterun/caml/major_gc.h:67: first defined here