- rename 'dir' to 'build_dir'
  - take 'package' from distribution option 'ext_package'
  - take 'extensions' from distribution option 'ext_modules'
  - take 'include_dirs' from distribution
Name keyword args explictly when calling CCompiler methods.
Overhauled how we generate extension filenames (in 'extension_filename()
  and 'build_extension()') to take 'package' option into account.