scons: Fix two problems with the way that the library path is generated.

scons: Fix two problems with the way that the library path is generated.
1) -L is automatically added, so don't do it ourselves
2) prepend the paths for gzstream and libelf so they are certain to
come first. The problem is that python might add /usr/lib to the path
and the user might have a locally installed version of libelf installed.