Libdwarf 20130729 (and later) support

Libdwarf 20130729 not only has changed `Dwarf_Callback_Func_c` definition, but also adds two new functions `dwarf_encode_leb128` and `dwarf_encode_signed_leb128`. So, we can check for a presence of symbol in a library, to distinguish between the old and new libdwarf. It is a little bit hackish, but was the best way I could figure out.

Fixes #1337.