I agree.
<prefix> for the current host build (whatever it is)
<prefix>/<subdir> for foreign multilibs
But that means different library binary packages for different build hosts.
For example, the mintlib binary package will have:
- for 68000 gcc, 68000 libraries in /usr/lib and ColdFire libraries in /usr/lib/m5475
- for ColdFire gcc, ColdFire libraries in /usr/lib and 68000 libraries in /usr/lib/m68000
In other words, if someone provides a binary package for a library, he will have to provide different packages for native 68000 gcc, native ColdFire gcc, Ubuntu cross-compiler, Cygwin cross-compiler... Every library binary package will contain exactly the same binaries, but in different directories.
I don't think this is a problem, since each distribution provides its own set of pre-compiled libraries: SpareMiNT for 68000, Cygwin cross-tools, future Gentoo for ColdFire, etc.