Slight change to the meaning of the 'libraries' list: if a library name

Slight change to the meaning of the 'libraries' list: if a library name
  has a directory component, then we only search for the library in
  that one directory, ie. ignore the 'library_dirs' lists for that
  one library.
Changed calling convention to 'gen_lib_options()' again: now, it takes
  a CCompiler instance and calls methods on it instead of taking
  format strings.  Also implemented the new "library name" semantics
  using the 'find_library_file()' method in the CCompiler instance.
Added 'force' flag to CCompiler; added to constructor and 'new_compiler()'.
Added 'warn()' method.