Allow building the demos to 'succeed' even if dune is absent.

GNUmakefile

@@ -128,7 +128,7 @@ package: clean
 	@ grep -v my_warnings src/_tags > $(PACKAGE)/src/_tags
 # Clean up the demos, including those that are not built by default
 # because they require dune.
-	@ $(MAKE) -C $(PACKAGE)/demos realclean
+	@ $(MAKE) -C $(PACKAGE)/demos clean
 # Set the version number into the files that mention it. These
 # include version.ml, StaticVersion.{ml,mli}, version.tex, META.
 	@ echo "-> Setting version to $(DATE)."

demos/Makefile

 # The following demos require menhirLib:
 #   calc-incremental
 #   calc-inspection
+
 # The following demos require menhirSdk:
 #   generate-printers
-# The following demos require dune (a.k.a. jbuilder)
-# and assume that Menhir is already installed:
-#   calc-dune
-#   calc-incremental-dune
+
+# The demos whose name ends in -dune require dune (a.k.a. jbuilder)
+# and assume that Menhir is installed.
 
 DEMOS := \
   calc \
@@ -15,8 +15,10 @@ DEMOS := \
   calc-incremental \
   calc-inspection \
   generate-printers \
+  calc-dune \
+  calc-incremental-dune \
 
-.PHONY: all clean realclean
+.PHONY: all clean
 
 all clean::
 	@for i in $(DEMOS) ; do \
@@ -25,7 +27,3 @@ all clean::
 
 clean::
 	/bin/rm -f *~ .*~
-
-realclean: clean
-	make -C calc-dune clean
-	make -C calc-incremental-dune clean

demos/calc-dune/Makefile

@@ -4,11 +4,15 @@ DUNE       := jbuilder
 EXECUTABLE := calc.exe
 
 all:
-	$(DUNE) build $(EXECUTABLE)
+	@ if command -v $(DUNE) > /dev/null ; then \
+	  $(DUNE) build $(EXECUTABLE) ; \
+	else \
+	  echo "Error: $(DUNE) is required." ; \
+	fi
 
 clean:
-	$(DUNE) clean
-	rm -f *~ .*~
+	rm -rf `cat .gitignore`
+	rm -f *~
 
 test: all
 	@echo "The following command should print 42:"

demos/calc-incremental-dune/Makefile

@@ -4,11 +4,15 @@ DUNE       := jbuilder
 EXECUTABLE := calc.exe
 
 all:
-	$(DUNE) build $(EXECUTABLE)
+	@ if command -v $(DUNE) > /dev/null ; then \
+	  $(DUNE) build $(EXECUTABLE) ; \
+	else \
+	  echo "Error: $(DUNE) is required." ; \
+	fi
 
 clean:
-	$(DUNE) clean
-	rm -f *~ .*~
+	rm -rf `cat .gitignore`
+	rm -f *~
 
 test: all
 	@echo "The following command should print 42:"