adjust Makefile and opam file for consistency with other projects

Makefile

+# Forward most targets to Coq makefile (with some trick to make this phony)
+%: Makefile.coq phony
+	+@make -f Makefile.coq $@
+
 all: Makefile.coq
-	@+$(MAKE) -f Makefile.coq all
+	+@make -f Makefile.coq all
+.PHONY: all
 
 clean: Makefile.coq
-	@+$(MAKE) -f Makefile.coq cleanall
-	@rm -f Makefile.coq Makefile.coq.conf
+	+@make -f Makefile.coq clean
+	find theories tests \( -name "*.d" -o -name "*.vo" -o -name "*.aux" -o -name "*.cache" -o -name "*.glob" -o -name "*.vio" \) -print -delete || true
+	rm -f Makefile.coq .lia.cache
+.PHONY: clean
+
+# Create Coq Makefile.
+Makefile.coq: _CoqProject Makefile
+	"$(COQBIN)coq_makefile" -f _CoqProject -o Makefile.coq
 
-Makefile.coq: _CoqProject
-	$(COQBIN)coq_makefile -f _CoqProject -o Makefile.coq
+# Install build-dependencies
+build-dep/opam: opam Makefile
+	@echo "# Creating build-dep package."
+	@mkdir -p build-dep
+	@sed <opam -E 's/^(build|install|remove):.*/\1: []/; s/^name: *"(.*)" */name: "\1-builddep"/' >build-dep/opam
+	@fgrep builddep build-dep/opam >/dev/null || (echo "sed failed to fix the package name" && exit 1) # sanity check
 
-force _CoqProject Makefile: ;
+build-dep: build-dep/opam phony
+	@# We want opam to not just instal the build-deps now, but to also keep satisfying these
+	@# constraints.  Otherwise, `opam upgrade` may well update some packages to versions
+	@# that are incompatible with our build requirements.
+	@# To achieve this, we create a fake opam package that has our build-dependencies as
+	@# dependencies, but does not actually install anything itself.
+	@echo "# Installing build-dep package."
+	@opam install $(OPAMFLAGS) build-dep/
 
-%: Makefile.coq force
-	@+$(MAKE) -f Makefile.coq $@
+# Some files that do *not* need to be forwarded to Makefile.coq
+Makefile: ;
+_CoqProject: ;
+opam: ;
 
-.PHONY: all clean force
\ No newline at end of file
+# Phony wildcard targets
+phony: ;
+.PHONY: phony

opam

@@ -10,12 +10,12 @@ version: "dev"
 
 synopsis: "Add support for Gallina names in intro patterns to the Iris Proof Mode"
 description: """
-This package implements string_to_ident in Ltac2, enabling support for Gallina names in Iris intro patterns (within strings)."""
+This package implements string_to_ident in Ltac2, enabling support for Gallina names in Iris intro patterns (within strings).
+"""
 
 depends: [
-  "ocaml" {>= "4.05.0"}
   "coq" {>= "8.11"}
-  "coq-iris" { (= "dev.2020-03-23.0.3a0d7152") | (= "dev") }
+  "coq-iris"
 ]
 
 build: [make "-j%{jobs}%"]