Make it possible to toggle NLS.

--- src/tint2conf/po/CMakeLists.txt.orig	2017-02-13 22:12:47 UTC
+++ src/tint2conf/po/CMakeLists.txt
@@ -1,4 +1,4 @@
-include(FindGettext)
+find_package(Gettext)
 if (GETTEXT_FOUND)
     set(GETTEXT_PACKAGE tint2conf)
     file(GLOB POTFILES RELATIVE ${CMAKE_CURRENT_SOURCE_DIR} "*.po")