Added 'extra_preargs' and 'extra_postargs' parameters to most methods,

Added 'extra_preargs' and 'extra_postargs' parameters to most methods,
  which allowed us to get rid of the 'build_info' used in some places
  (a temporary kludge to support MSVC++ "def" files).
Deleted big comment whining about that kludge.
Added 'compiler_type' class attribute.
Overhauled 'new_compiler()': now takes 'compiler' argument along with
  'plat' (both optional with sensible defaults), and looks them both up
  in the new 'default_compiler' and 'compiler_class' dictionaries to
  figure out where to get the concrete compiler class from.
Reordered arguments to 'gen_lib_options()' to match the order in
  which the arguments are generated (ie. -L before -l).