Build fails for unlinkable_fobjects across windows drive boundaries

Found from a CI run:

  Running from numpy source directory.
  c:\hostedtoolcache\windows\python\3.6.8\x64\lib\distutils\dist.py:261: UserWarning: Unknown distribution option: 'define_macros'
    warnings.warn(msg)
  Traceback (most recent call last):
    File "c:\hostedtoolcache\windows\python\3.6.8\x64\lib\site-packages\pip\_vendor\pep517\_in_process.py", line 280, in <module>
      main()
    File "c:\hostedtoolcache\windows\python\3.6.8\x64\lib\site-packages\pip\_vendor\pep517\_in_process.py", line 263, in main
      json_out['return_val'] = hook(**hook_input['kwargs'])
    File "c:\hostedtoolcache\windows\python\3.6.8\x64\lib\site-packages\pip\_vendor\pep517\_in_process.py", line 205, in build_wheel
      metadata_directory)
    File "C:\Users\VSSADM~1\AppData\Local\Temp\pip-build-env-4zqwivu4\overlay\Lib\site-packages\setuptools\build_meta.py", line 213, in build_wheel
      wheel_directory, config_settings)
    File "C:\Users\VSSADM~1\AppData\Local\Temp\pip-build-env-4zqwivu4\overlay\Lib\site-packages\setuptools\build_meta.py", line 198, in _build_with_temp_dir
      self.run_setup()
    File "C:\Users\VSSADM~1\AppData\Local\Temp\pip-build-env-4zqwivu4\overlay\Lib\site-packages\setuptools\build_meta.py", line 250, in run_setup
      self).run_setup(setup_script=setup_script)
    File "C:\Users\VSSADM~1\AppData\Local\Temp\pip-build-env-4zqwivu4\overlay\Lib\site-packages\setuptools\build_meta.py", line 143, in run_setup
      exec(compile(code, __file__, 'exec'), locals())
    File "setup.py", line 488, in <module>
      setup_package()
    File "setup.py", line 480, in setup_package
      setup(**metadata)
    File "D:\a\1\s\numpy\distutils\core.py", line 169, in setup
      return old_setup(**new_attr)
    File "C:\Users\VSSADM~1\AppData\Local\Temp\pip-build-env-4zqwivu4\overlay\Lib\site-packages\setuptools\__init__.py", line 144, in setup
      return distutils.core.setup(**attrs)
    File "c:\hostedtoolcache\windows\python\3.6.8\x64\lib\distutils\core.py", line 148, in setup
      dist.run_commands()
    File "c:\hostedtoolcache\windows\python\3.6.8\x64\lib\distutils\dist.py", line 955, in run_commands
      self.run_command(cmd)
    File "c:\hostedtoolcache\windows\python\3.6.8\x64\lib\distutils\dist.py", line 974, in run_command
      cmd_obj.run()
    File "C:\Users\VSSADM~1\AppData\Local\Temp\pip-build-env-4zqwivu4\overlay\Lib\site-packages\wheel\bdist_wheel.py", line 223, in run
      self.run_command('build')
    File "c:\hostedtoolcache\windows\python\3.6.8\x64\lib\distutils\cmd.py", line 313, in run_command
      self.distribution.run_command(command)
    File "c:\hostedtoolcache\windows\python\3.6.8\x64\lib\distutils\dist.py", line 974, in run_command
      cmd_obj.run()
    File "D:\a\1\s\numpy\distutils\command\build.py", line 40, in run
      old_build.run(self)
    File "c:\hostedtoolcache\windows\python\3.6.8\x64\lib\distutils\command\build.py", line 135, in run
      self.run_command(cmd_name)
    File "c:\hostedtoolcache\windows\python\3.6.8\x64\lib\distutils\cmd.py", line 313, in run_command
      self.distribution.run_command(command)
    File "c:\hostedtoolcache\windows\python\3.6.8\x64\lib\distutils\dist.py", line 974, in run_command
      cmd_obj.run()
    File "D:\a\1\s\numpy\distutils\command\build_ext.py", line 272, in run
      self.build_extensions()
    File "c:\hostedtoolcache\windows\python\3.6.8\x64\lib\distutils\command\build_ext.py", line 448, in build_extensions
      self._build_extensions_serial()
    File "c:\hostedtoolcache\windows\python\3.6.8\x64\lib\distutils\command\build_ext.py", line 473, in _build_extensions_serial
      self.build_extension(ext)
    File "setup.py", line 260, in build_extension
      build_ext.build_extension(self, ext)
    File "D:\a\1\s\numpy\distutils\command\build_ext.py", line 478, in build_extension
      unlinkable_fobjects)
    File "D:\a\1\s\numpy\distutils\command\build_ext.py", line 523, in _process_unlinkable_fobjects
      fobjects = [os.path.relpath(obj) for obj in unlinkable_fobjects]
    File "D:\a\1\s\numpy\distutils\command\build_ext.py", line 523, in <listcomp>
      fobjects = [os.path.relpath(obj) for obj in unlinkable_fobjects]
    File "c:\hostedtoolcache\windows\python\3.6.8\x64\lib\ntpath.py", line 584, in relpath
      path_drive, start_drive))
  ValueError: path is on mount 'c:', start on mount 'D:'
  ERROR: Failed building wheel for numpy
  Building wheel for numpy (PEP 517): finished with status 'error'
Failed to build numpy
ERROR: Failed to build one or more wheels
Exception information:
Traceback (most recent call last):
  File "c:\hostedtoolcache\windows\python\3.6.8\x64\lib\site-packages\pip\_internal\cli\base_command.py", line 188, in _main
    status = self.run(options, args)
  File "c:\hostedtoolcache\windows\python\3.6.8\x64\lib\site-packages\pip\_internal\cli\req_command.py", line 185, in wrapper
    return func(self, options, args)
  File "c:\hostedtoolcache\windows\python\3.6.8\x64\lib\site-packages\pip\_internal\commands\wheel.py", line 189, in run
    "Failed to build one or more wheels"
pip._internal.exceptions.CommandError: Failed to build one or more wheels

[eric-wieser]

Duplicate of #12530

[mattip]

One problem with absolute paths may be feeding them to a migw-based compiler, so tests to prove this works will be needed