APP_LIB(XX)_FILES contains some files more than once, this causes the following cp warning
  cp: warning: source file `X' specified more than once
  cp: warning: source file `Y' specified more than once
  cp: warning: source file `Z' specified more than once

Silence it by using sort (sorts and removes duplicates).
--- Makefile
+++ Makefile
@@ -107,8 +107,8 @@
 
 install:
 	mkdir -p $(DESTDIR)$(libdir)/
-#	cp -af $(APP_LIB_FILES) $(DESTDIR)$(libdir)/
-	cp -af $(APP_LIBXX_FILES) $(DESTDIR)$(libdir)/
+#	cp -af $(sort $(APP_LIB_FILES)) $(DESTDIR)$(libdir)/
+	cp -af $(sort $(APP_LIBXX_FILES)) $(DESTDIR)$(libdir)/
 	mkdir -p $(DESTDIR)$(includedir)/
 	for d in pjlib pjlib-util pjnath pjmedia pjsip; do \
 		cp -RLf $$d/include/* $(DESTDIR)$(includedir)/; \