Force building standalone library (e.g., put headers in appropriate place).

Index: lib/tevent/wscript
--- lib/tevent/wscript.orig
+++ lib/tevent/wscript
@@ -42,6 +42,7 @@ def configure(conf):
             conf.define('USING_SYSTEM_CMOCKA', 1)
 
     conf.env.standalone_tevent = conf.IN_LAUNCH_DIR()
+    conf.env.standalone_tevent = True
 
     if not conf.env.standalone_tevent:
         if conf.CHECK_BUNDLED_SYSTEM_PKG('tevent', minversion=VERSION,