Fix library name and path on some 64bit systems

This contains minor tweak against fix_64 I have send few day ago.

This patch fix library name and path by correcting handling LIB_PREFIX variable. This variable should be used as library directory suffix, not library name suffix. On systems where LIB_PREFIX is not empty the library name is changed, but tests are linked against '-lswiftnaw', no name modification is performed.

This bug mostly affects packaging fox distributions.