Fix clang -Wstrict-prototypes warnings
(I was going to just submit a patch, but trying to fork the repo says "You have reached your project limit". Not sure what that means...)
It would be nice to fix all clang -Wstrict-prototypes warnings because the clang people are thinking to enable that warning by default and also make it an error by default:
https://discourse.llvm.org/t/rfc-enabling-wstrict-prototypes-by-default-in-c/60521
There is at least one warning from gl2ps, which I noticed trying to build VTK with -Wstrict-prototypes.