`tests/test_pkgconfig.py::test_configure_extension` is failing on lib64 systems

[mgorny]

When running on a modern 64-bit system with lib64 directory, the following test fails:

============================================================== FAILURES ===============================================================
______________________________________________________ test_configure_extension _______________________________________________________

    def test_configure_extension():
        ext = Extension('foo', ['foo.c'])
        pkgconfig.configure_extension(ext, 'fake-gtk+-3.0 fake-python')
        assert sorted(ext.extra_compile_args) == [
             '-DGSEAL_ENABLE', '-I/usr/include/gtk-3.0', '-I/usr/include/python2.7']
        assert sorted(ext.extra_link_args) == [
            '-L/usr/lib_gtk_foo', '-L/usr/lib_python_foo', '-lgtk-3', '-lpython2.7']
    
        ext = Extension('foo', ['foo.c'])
        pkgconfig.configure_extension(ext, 'fake-ldonly-pkg')
        assert ext.extra_compile_args == []
>       assert ext.extra_link_args == ['-lfakeldonly']
E       AssertionError: assert ['-L/usr/lib', '-lfakeldonly'] == ['-lfakeldonly']
E         
E         At index 0 diff: '-L/usr/lib' != '-lfakeldonly'
E         Left contains one more item: '-lfakeldonly'
E         
E         Full diff:
E           [
E         +     '-L/usr/lib',
E               '-lfakeldonly',
E           ]

tests/test_pkgconfig.py:150: AssertionError
==================================================== 1 failed, 34 passed in 0.53s =====================================================

I can reproduce via running tox on Gentoo/amd64, on top of a97fa85.