(bootstrap-temacs): Warn that parts of commands enclosed in $(ARGQUOTE)s

should not be split between two lines, as that will break with GNU Make >3.80,
when sh.exe is used and arg quoting is with '..'.

src/makefile.w32-in

@@ -182,6 +182,8 @@ bootstrap: bootstrap-emacs
 # Build a temacs with a sufficiently large PURESIZE to load the
 # Lisp files from loadup.el in source form.
 #
+# WARNING: Do NOT split the part inside $(ARGQUOTE)s into multiple lines as
+#          this can break with GNU Make 3.81 and later if sh.exe is used.
 bootstrap-temacs:
 	$(MAKE) $(MFLAGS) temacs CFLAGS=$(ARGQUOTE)$(CFLAGS) -DPURESIZE=5000000$(ARGQUOTE)