ppci-ld: linker imports too many modules from the libraries

[tstreiff]

I have been using the --library option of the linker for several weeks.
I sometimes got unexpected errors "Multiple defined symbol" (most often "errno") I failed to explain.
I noticed that the linker sometimes merges library object files that only uses unresolved symboles but do not define them. This sometimes leads to attempt to merge several times the same object file, hence the error reports.

When it tries to complete unresolved references with library object files, the linker merges an object file if at least one program unresolved symbol is listed in this object file. But it does not check that this symbol is defined in this object file, the symbol can be only be used.

In linker.py/add_missing_symbols_from_libraries() checks if an object file should be merged as follows:

has_sym = any(map(obj.has_symbol, undefined_symbols))

it should also test that the symbol is global and defined.