BLD: distutils does not gracefully handle separate Windows Drive Paths #12530
Description
Building NumPy from source on a Windows path that starts from a different drive letter from the library paths causes a traceback as shown below. One might generally assume this is a bad idea, but it is the exact scenario currently present for Windows images on Azure (I use the -n flag to runtests.py to circumvent this, but this may prevent the junit xml file from containing build info, which may be causing one of the two Windows warnings observed there at the moment; the other warning I've patched in my fork already).
2018-12-11T03:39:18.5547666Z Traceback (most recent call last):
2018-12-11T03:39:18.5548033Z File "setup.py", line 411, in <module>
2018-12-11T03:39:18.5548537Z setup_package()
2018-12-11T03:39:18.5548982Z File "setup.py", line 403, in setup_package
2018-12-11T03:39:18.5549353Z setup(**metadata)
2018-12-11T03:39:18.5549758Z File "D:\a\1\s\numpy\distutils\core.py", line 171, in setup
2018-12-11T03:39:18.5550129Z return old_setup(**new_attr)
2018-12-11T03:39:18.5550540Z File "C:\hostedtoolcache\windows\Python\3.7.0\x64\lib\site-packages\setuptools\__init__.py", line 143, in setup
2018-12-11T03:39:18.5551173Z return distutils.core.setup(**attrs)
2018-12-11T03:39:18.5551590Z File "C:\hostedtoolcache\windows\Python\3.7.0\x64\lib\distutils\core.py", line 148, in setup
2018-12-11T03:39:18.5551960Z dist.run_commands()
2018-12-11T03:39:18.5552336Z File "C:\hostedtoolcache\windows\Python\3.7.0\x64\lib\distutils\dist.py", line 966, in run_commands
2018-12-11T03:39:18.5552725Z self.run_command(cmd)
2018-12-11T03:39:18.5553189Z File "C:\hostedtoolcache\windows\Python\3.7.0\x64\lib\distutils\dist.py", line 985, in run_command
2018-12-11T03:39:18.5553718Z cmd_obj.run()
2018-12-11T03:39:18.5554097Z File "D:\a\1\s\numpy\distutils\command\build.py", line 47, in run
2018-12-11T03:39:18.5554485Z old_build.run(self)
2018-12-11T03:39:18.5554860Z File "C:\hostedtoolcache\windows\Python\3.7.0\x64\lib\distutils\command\build.py", line 135, in run
2018-12-11T03:39:18.5555342Z self.run_command(cmd_name)
2018-12-11T03:39:18.5555739Z File "C:\hostedtoolcache\windows\Python\3.7.0\x64\lib\distutils\cmd.py", line 313, in run_command
2018-12-11T03:39:18.5556146Z self.distribution.run_command(command)
2018-12-11T03:39:18.5556527Z File "C:\hostedtoolcache\windows\Python\3.7.0\x64\lib\distutils\dist.py", line 985, in run_command
2018-12-11T03:39:18.5556912Z cmd_obj.run()
2018-12-11T03:39:18.5557283Z File "D:\a\1\s\numpy\distutils\command\build_ext.py", line 261, in run
2018-12-11T03:39:18.5557669Z self.build_extensions()
2018-12-11T03:39:18.5558066Z File "C:\hostedtoolcache\windows\Python\3.7.0\x64\lib\distutils\command\build_ext.py", line 448, in build_extensions
2018-12-11T03:39:18.5558439Z self._build_extensions_serial()
2018-12-11T03:39:18.5558844Z File "C:\hostedtoolcache\windows\Python\3.7.0\x64\lib\distutils\command\build_ext.py", line 473, in _build_extensions_serial
2018-12-11T03:39:18.5559251Z self.build_extension(ext)
2018-12-11T03:39:18.5559627Z File "D:\a\1\s\numpy\distutils\command\build_ext.py", line 467, in build_extension
2018-12-11T03:39:18.5560009Z unlinkable_fobjects)
2018-12-11T03:39:18.5560406Z File "D:\a\1\s\numpy\distutils\command\build_ext.py", line 512, in _process_unlinkable_fobjects
2018-12-11T03:39:18.5560802Z fobjects = [os.path.relpath(obj) for obj in unlinkable_fobjects]
2018-12-11T03:39:18.5561184Z File "D:\a\1\s\numpy\distutils\command\build_ext.py", line 512, in <listcomp>
2018-12-11T03:39:18.5561561Z fobjects = [os.path.relpath(obj) for obj in unlinkable_fobjects]
2018-12-11T03:39:18.5561974Z File "C:\hostedtoolcache\windows\Python\3.7.0\x64\lib\ntpath.py", line 564, in relpath
2018-12-11T03:39:18.5562363Z path_drive, start_drive))
2018-12-11T03:39:18.5562726Z ValueError: path is on mount 'C:', start on mount 'D:'