rbconfig/sizeof: Add VPATH for nmake

Author: nobu

ext/rbconfig/sizeof/depend

@@ -1,10 +1,10 @@
 # sources
 
-limits.c: $(top_srcdir)/tool/generic_erb.rb $(top_srcdir)/template/limits.c.tmpl
+{$(VPATH)}limits.c: $(top_srcdir)/tool/generic_erb.rb $(top_srcdir)/template/limits.c.tmpl
 	$(Q) $(RUBY) $(top_srcdir)/tool/generic_erb.rb --output=$@ \
 		$(top_srcdir)/template/limits.c.tmpl \
 
-sizes.c: $(top_srcdir)/tool/generic_erb.rb \
+{$(VPATH)}sizes.c: $(top_srcdir)/tool/generic_erb.rb \
 		   $(top_srcdir)/template/sizes.c.tmpl \
 		   $(top_srcdir)/configure.ac \
 		   $(top_srcdir)/ext/rbconfig/sizeof/extconf.rb