DT_RUNPATH, DT_RPATH, LD_PRELOAD, LD_AUDIT, etc. can use `$LIB` which
is expanded to the relative path to libdir from prefix.  However glibc
was expanding `$LIB` as e.g. `x86-64-linux-gnu` instead of
`lib/x86-64-linux-gnu`.

Fixes #874