patch 8.1.2045: the option.c file is too big

Problem:    The option.c file is too big.
Solution:   Split off the code dealing with strings. (Yegappan Lakshmanan,
            closes #4937)
diff --git a/src/Make_mvc.mak b/src/Make_mvc.mak
index 7aa7ef8..1205a6e 100644
--- a/src/Make_mvc.mak
+++ b/src/Make_mvc.mak
@@ -764,6 +764,7 @@
 	$(OUTDIR)\normal.obj \
 	$(OUTDIR)\ops.obj \
 	$(OUTDIR)\option.obj \
+	$(OUTDIR)\optionstr.obj \
 	$(OUTDIR)\os_mswin.obj \
 	$(OUTDIR)\os_win32.obj \
 	$(OUTDIR)\pathdef.obj \
@@ -1606,6 +1607,8 @@
 
 $(OUTDIR)/option.obj:	$(OUTDIR) option.c  $(INCL) optiondefs.h
 
+$(OUTDIR)/optionstr.obj:	$(OUTDIR) optionstr.c  $(INCL)
+
 $(OUTDIR)/ops.obj:	$(OUTDIR) ops.c  $(INCL)
 
 $(OUTDIR)/os_mswin.obj:	$(OUTDIR) os_mswin.c  $(INCL)
@@ -1816,6 +1819,7 @@
 	proto/normal.pro \
 	proto/ops.pro \
 	proto/option.pro \
+	proto/optionstr.pro \
 	proto/os_mswin.pro \
 	proto/winclip.pro \
 	proto/os_win32.pro \