Added big comment about the kludginess of passing 'build_options'
to the link methods and how to fix it.
Added 'gen_preprocess_options()' and 'gen_lib_options()' convenience
functions -- the two cases are very similar for Unix C Compilers and
VC++, so I figured I might as well unify the implementations.
- fix some broken abstract methods
- kludge: add 'build_info' parameter to link methods
- add 'object_name()' and 'shared_library_name()'
- support for MSVCCompiler class on NT/Win95
'new_compiler()' factory function.
Added 'runtime_library_dirs' list (for -R linker option) and methods
to manipulate it.
Deleted some obsolete comments.
Added all the filename manglign methods: 'object_filenames()',
'shared_object_filename()', 'library_filename()',
'shared_library_filename()'.
Added 'spawn()' method (front end to the "real" spawn).