modules: use libtool object files - real object file might be in a .libs subdir

Build failed with:

    g++-4.8: error: ../modules/gpgsqlbackend/gpgsqlbackend.o: No such file or directory
    g++-4.8: error: ../modules/gpgsqlbackend/spgsql.o: No such file or directory

Searching object files through the libtool object files instead worked.