Fix 'make distcheck' by removing the manpage additional generated .3 files

Part One:

Makefile.am:

Correct the name of the libcoap-*.pc in distclean-local: of Makefile.am

Makefile.am

@@ -233,7 +233,7 @@ distclean-local:
 	@rm -f src/*.o src/*.lo
 	rm -f libcoap-$(LIBCOAP_API_VERSION).map
 	rm -f libcoap-$(LIBCOAP_API_VERSION).sym
-	rm -f libcoap-$(LIBCOAP_API_VERSION).pc
+	rm -f libcoap-$(LIBCOAP_NAME_SUFFIX).pc
 	@echo
 	@echo "     ---> Please note the following important advice! <---"
 	@echo "     The files libcoap-$(LIBCOAP_API_VERSION).{map,sym} are removed by the distclean target!"