coq-makefile: generate combined rules for .vo and .glob
coq-makefile: generate combined rules for .vo and .glob

Without this, running `make -j 2 html` could run coqc multiple times
on a file, see eg
https://coq.zulipchat.com/#narrow/stream/237656-Coq-devs-.26-plugin-devs/topic/Removed.20file.20warning

SkySkimmer/coq,SkySkimmer/coq,SkySkimmer/coq,SkySkimmer/coq