Add `make install DESTDIR=...` support

--- db-4.1.25/dist/Makefile.in
+++ db-4.1.25/dist/Makefile.in
@@ -548,10 +548,10 @@
 INCINC=	cxx_common.h cxx_except.h
 install_include:
 	@echo "Installing DB include files: $(includedir)/STLdb4 ..."
-	@test -d $(includedir)/STLdb4 || \
-	    ($(mkdir) -p $(includedir)/STLdb4 && $(chmod) $(dmode) $(includedir)/STLdb4)
-	@cd $(includedir)/STLdb4 && $(rm) -f stldbraw.h
-	@$(cp) -p stldbraw.h $(includedir)/STLdb4
+	@test -d $(DESTDIR)$(includedir)/STLdb4 || \
+	    ($(mkdir) -p $(DESTDIR)$(includedir)/STLdb4 && $(chmod) $(dmode) $(DESTDIR)$(includedir)/STLdb4)
+	@cd $(DESTDIR)$(includedir)/STLdb4 && $(rm) -f stldbraw.h
+	@$(cp) -p stldbraw.h $(DESTDIR)$(includedir)/STLdb4
 
 uninstall_include:
 	@cd $(includedir)/STLdb4 && $(rm) -f stldbraw.h