The no git code also check for an existing version.h

My previous patch always used an empty file if git was not
available. This patch extends this to use the existing
version.h file if it exists (snapshot, etc.)

Makefile.in

@@ -188,7 +188,13 @@ iverilog-vpi.pdf: iverilog-vpi.ps
 .PHONY: version.h
 version.h:
 ifeq ($(GIT),none)
-	@echo '#define VERSION_TAG ""' > $@;
+	@if test -r $(srcdir)/$@; then \
+	    echo "Using $(srcdir)/$@ for VERSION_TAG"; \
+	    diff $(srcdir)/$@ $@ > /dev/null 2>&1 || cp $(srcdir)/$@ $@; \
+	else \
+	    echo "Using empty VERSION_TAG"; \
+	    echo '#define VERSION_TAG ""' > $@; \
+	fi
 else
 	@if test -d $(srcdir)/.git; then \
 	    echo "Using git-describe for VERSION_TAG"; \