patch 8.1.2005: the regexp.c file is too big

Problem:    The regexp.c file is too big.
Solution:   Move the backtracking engine to a separate file. (Yegappan
            Lakshmanan, closes #4905)
diff --git a/src/Make_mvc.mak b/src/Make_mvc.mak
index 0401902..ac02e80 100644
--- a/src/Make_mvc.mak
+++ b/src/Make_mvc.mak
@@ -1637,7 +1637,7 @@
 
 $(OUTDIR)/quickfix.obj:	$(OUTDIR) quickfix.c  $(INCL)
 
-$(OUTDIR)/regexp.obj:	$(OUTDIR) regexp.c regexp_nfa.c  $(INCL)
+$(OUTDIR)/regexp.obj:	$(OUTDIR) regexp.c regexp_bt.c regexp_nfa.c  $(INCL)
 
 $(OUTDIR)/scriptfile.obj:	$(OUTDIR) scriptfile.c  $(INCL)