Clarify the way make runtop works.

This also introduces a bit of refactoring between the runtop and
natruntop targets, ensuring both toplevels are called with the same
command-line options.

Makefile

@@ -82,12 +82,6 @@ LIBFILES=stdlib.cma std_exit.cmo *.cmi camlheader
 COMPLIBDIR=$(LIBDIR)/compiler-libs
 
 TOPINCLUDES=$(addprefix -I otherlibs/,$(filter-out %threads,$(OTHERLIBRARIES)))
-RUNTOP = $(OCAMLRUN) ./ocaml$(EXE) \
-  -nostdlib -I stdlib -I toplevel \
-  -noinit $(TOPFLAGS) $(TOPINCLUDES)
-NATRUNTOP=./ocamlnat$(EXE) \
-  -nostdlib -I stdlib -I toplevel \
-  -noinit $(TOPFLAGS) $(TOPINCLUDES)
 ifeq "$(UNIX_OR_WIN32)" "unix"
 EXTRAPATH=
 else
@@ -626,20 +620,28 @@ ocaml$(EXE): $(expunge) ocaml.tmp
 partialclean::
 	rm -f ocaml$(EXE)
 
+# Use TOPFLAGS to pass additional flags to the bytecode or native toplevel
+# when running make runtop or make natruntop
+TOPFLAGS ?=
+OC_TOPFLAGS = -nostdlib -I stdlib -I toplevel -noinit $(TOPINCLUDES) $(TOPFLAGS)
+
+# Note: Beware that, since this rule begins with a coldstart, both
+# boot/ocamlrun and runtime/ocamlrun will be the same when the toplevel
+# is run.
 .PHONY: runtop
 runtop:
 	$(MAKE) coldstart
 	$(MAKE) ocamlc
 	$(MAKE) otherlibraries
 	$(MAKE) ocaml
-	@$(EXTRAPATH) $(RLWRAP) $(RUNTOP)
+	@$(EXTRAPATH) $(RLWRAP) $(OCAMLRUN) ./ocaml$(EXE) $(OC_TOPFLAGS)
 
 .PHONY: natruntop
 natruntop:
 	$(MAKE) core
 	$(MAKE) opt
 	$(MAKE) ocamlnat
-	@$(FLEXLINK_ENV) $(EXTRAPATH) $(RLWRAP) $(NATRUNTOP)
+	@$(FLEXLINK_ENV) $(EXTRAPATH) $(RLWRAP) ./ocamlnat$(EXE) $(OC_TOPFLAGS)
 
 # Native dynlink