Fix libctf build on non-ELF targets.

Fix libctf build on non-ELF targets.

All machinery works as on ELF, except for automatic loading of ELF
string and symbol tables in the BFD-style open machinery.

        * Makefile.def (dependencies): configure-libctf depends on all-bfd
        and all its deps.
        * Makefile.in: Regenerated.

libctf/
        * configure.in: Check for bfd_section_from_elf_index.
        * configure: Regenerate.
        * config.h.in [HAVE_BFD_ELF]: Likewise.
        * libctf/ctf_open_bfd (ctf_bfdopen_ctfsect): Use it.
        abfd is potentially unused now.