Fix scons building under unix. Now we should really have only *one* '/usr/lib' lib search path, in last position! Previously we could have a bunch of those, even in first position, which would e.g. prevent linking against a custom boost in /opt/lib/boost if you had a system boost installed...

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54230 954f8c5b-7b00-dc11-b283-0030488c597c