Now that LIBRARY is the versioned name (e.g. "libunicorn.so.0"), we
don't need this rename step; just create the symlink for libunicorn.so.
If VERSION_EXT isn't set, then the library we installed already has the
correct name, and no symlink is necessary.
We should probably be running 'ldconfig' to handle this, or better yet,
using libtool.
This allows executables (e.g. unit tests) to link against the
libunicorn.so in the build directory, after the SONAME was added in
4860fdb3.
Becuase libunicorn.so has an SONAME of libunicorn.so.0, the linker uses
the SONAME for the DT_NEEDED entry of the tests. But because a library
with that name did not exist, they would fail to run.
