Merge pull request #492 from mtzguido/noinc

Makefile: improve confusing errors in inclusion

Makefile

@@ -1,5 +1,16 @@
+# Including a Makefile from the visitors package, but making sure
+# to give decent errors.
+
 # make src/Ast.processed.ml
-include $(shell ocamlfind query visitors)/Makefile.preprocess
+_:=$(shell ocamlfind query)
+ifneq ($(.SHELLSTATUS),0)
+_: $(error "'ocamlfind query' failed, please install OCaml and put it in your PATH)
+endif
+visitors_root:=$(shell ocamlfind query visitors)
+ifneq ($(.SHELLSTATUS),0)
+_: $(error "'ocamlfind query visitors' failed, please 'opam install visitors')
+endif
+include $(visitors_root)/Makefile.preprocess
 
 .PHONY: all minimal clean test pre krmllib install