old_contents: from distutils.core import Extension
  from os.path import dirname, join
  
  
  def get_extensions():
      ROOT = dirname(__file__)
  
      return [
          Extension('astropy.utils._compiler',
                    [join(ROOT, 'src', 'compiler.c')])
          ]
  
new_contents: from distutils.core import Extension
  from os.path import dirname, join, relpath
  
  
  def get_extensions():
      ROOT = dirname(__file__)
  
      return [
          Extension('astropy.utils._compiler',
                    [relpath(join(ROOT, 'src', 'compiler.c'))])
          ]
  
subject: Make sure to use the relative path for all C extension source files. Otherwise distuils' MSVC compiler generates some potentially long (too long for Windows) pathnames in the build\temp dir for various compiler artifacts. (This was particularly problematic in Jenkins, where having multiple configuration matrix axes can make for long path names.)