cpp: enable as many warnings as practical

@@ -31,11 +31,15 @@
         y.tab.o$(STRAP)
 
 CLEANFILES += \
         y.tab.c$(STRAP)
 
-CFLAGS +=       -O2
+CERRWARN=       -Wall -Wextra
+CERRWARN +=     -Wno-unknown-pragmas
+CERRWARN +=     -Wno-sign-compare
+CERRWARN +=     -Wno-unused-label
+CFLAGS +=       -O2 $(CERRWARN)
 LD =            $(GCC)
 
 COMPILE.c =     $(GCC) $(CPPFLAGS) $(CFLAGS) -c $< -o $@
 LINK.prog =     $(LD) $(LDFLAGS) -o $@ $(OBJS) $(LIBS)
 PROTOFILES =    $(DESTDIR)$(PROGDIR)/$(PROG)