[build] prevent bad sed usage under osx

lib/Makefile.am

@@ -73,7 +73,7 @@ clean-local:
 
 install-data-local:
 if MACOS
-	${SED} -i '' -e "s|(DESTDIR)|${DESTDIR}|g" $(pcfiles)
+	sed -i '' -e "s|(DESTDIR)|${DESTDIR}|g" $(pcfiles)
 else
 	${SED} -i "s|(DESTDIR)|${DESTDIR}|g" $(pcfiles)
 endif