fix Makefile; fix coq-stdpp dep

Makefile

@@ -41,6 +41,7 @@ build-dep: build-dep/opam phony
 Makefile: ;
 _CoqProject: ;
 awk.Makefile: ;
+opam: ;
 
 # Phony wildcard targets
 phony: ;

opam

@@ -12,5 +12,5 @@ remove: ["sh" "-c" "rm -rf '%{lib}%/coq/user-contrib/iris'"]
 depends: [
   "coq" { >= "8.6.1" & < "8.8~" }
   "coq-mathcomp-ssreflect" { (>= "1.6.1" & < "1.7~") | (= "dev") }
-  "coq-stdpp" { (= "2017-09-21.2") | (= "dev") }
+  "coq-stdpp" { (= "dev.2017-09-21.2") | (= "dev") }
 ]