On HP-UX machines some system libraries appear in architecture-specific
implicit linker search paths.  We need to add these paths to our system
library search path.  However, at the time we construct the search path
we do not know the target architecture.

A full solution requires re-organizing platform configuration files so
that the target architecture can be known when needed.  Until that
happens we can avoid the problem by searching in both 32-bit and 64-bit
implicit link directories.  By telling CMake that they are implicit
directories the generated link lines will never pass the paths, leaving
the linker free to find the library of the proper architecture even if
the find_library call finds the wrong one.