- add 'extra_preargs' and 'extra_postargs' parameters (and use them!)
  - got rid of 'build_info' kludge parameter
  - added 'compiler_type' class attribute
  - respect reordered arguments to 'gen_lib_options()'
Also added 'output_dir' parameter (catching up with older change in
  CCompiler) -- BUT this is presently ignored by all methods!
Deleted some more docstrings redundant with CCompiler.
Dropped generated of "/DEF:" argument --- that's now done by
  the 'build_ext' command.