In the MinGW iverilog-vpi driver use $CFLAGS instead of CXXFLAGS.

CFLAGS has the appropriate -W flags defined while CXXFLAGS is
just the default. The script iverilog-vpi uses CXXFLAGS because
it is defined in the Makefile to have the -W flags. This
directory is C based so does not define CXXFLAGS.

driver-vpi/Makefile.in

@@ -72,7 +72,7 @@ main.o: main.c config.h
 config.h: $(srcdir)/config.h.in Makefile
 	sed -e 's;@IVLCC@;@CC@;' -e 's;@IVLCXX@;@CXX@;' \
 	    -e 's;@SUFFIX@;$(suffix);g' \
-	    -e 's;@IVLCFLAGS@;@CXXFLAGS@;' \
+	    -e 's;@IVLCFLAGS@;$(CFLAGS);' \
 	    -e 's;@SHARED@;@shared@;' $< > $@
 
 # Windows specific...