shill: vpn: Use LIBDIR passed down from ebuild

Send LIBDIR parameter down to a macro so consumers like OpenVPNDriver
can configure pathnames correctly.  Also pass SYSROOT down into unit
tests so we can verify that the path dependencies exist in the image.

BUG=chromium-os:27718
TEST=New unittest; strings

Change-Id: I63ba227923fdfb51a7ea9f5b17234104bd7502f9
Reviewed-on: https://gerrit.chromium.org/gerrit/18209
Commit-Ready: Paul Stewart <pstew@chromium.org>
Reviewed-by: Paul Stewart <pstew@chromium.org>
Tested-by: Paul Stewart <pstew@chromium.org>
diff --git a/openvpn_driver.h b/openvpn_driver.h
index 39c8502..66eb092 100644
--- a/openvpn_driver.h
+++ b/openvpn_driver.h
@@ -72,6 +72,10 @@
   FRIEND_TEST(OpenVPNDriverTest, ParseRouteOption);
   FRIEND_TEST(OpenVPNDriverTest, SetRoutes);
   FRIEND_TEST(OpenVPNDriverTest, SpawnOpenVPN);
+  FRIEND_TEST(OpenVPNDriverTest, VerifyPaths);
+
+  static const char kOpenVPNPath[];
+  static const char kOpenVPNScript[];
 
   // The map is a sorted container that allows us to iterate through the options
   // in order.