Fix issue 151: SCons env['LIBPATH'] sometimes is a string and that breaks RPATH.
Fix issue 151: SCons env['LIBPATH'] sometimes is a string and that breaks RPATH.

Patch by: mgorny at gentoo.org (slightly modified).

* SConstruct: Always append a list to environment variables, to guarantee that
    the outcome is a list so further appends will add the new values to a list.

jandre/serf,jandre/serf,jandre/serf,jandre/serf