Prevent underlinking issues due to recent boost versions requiring boost::system
See also: https://bugs.gentoo.org/show_bug.cgi?id=579398

--- lvtk-1.2.0/wscript
+++ lvtk-1.2.0/wscript
@@ -71,6 +71,7 @@
                             atleast_version="2.20.0", mandatory=False)
 
     # Setup the Environment
+    conf.env.LIB_BOOST = ['boost_system']
     conf.env.EXAMPLES_DISABLED  = conf.options.disable_examples
     conf.env.TOOLS_DISABLED     = conf.options.disable_tools
     conf.env.UI_DISABLED        = conf.options.disable_ui