libstdc++ multilibs installed where gcc won't find them

[David Edelsohn]

	As Albert Chin points out, the following line in zlib configure
assumes CC is GCC:

toolexeclibdir=$toolexeclibdir/`$CC -print-multi-os-directory 2>/dev/null || echo .`

This assignment is fine when zlib is built as a target library, but not as
a build library using the native compiler at bootstrap.  All platforms
using a non-GCC compiler to boostrap will have problems.

David