Set first_system_include for first system header directory.
(is_system_include): Start search at first_system_include.

(INCLUDE_LEN_FUDGE): Defined.
(append_include_chain): New function.  Adds directory(ies)
to include chain and updates value of max_include_len.
(main, path_include): Use append_include_chain.
(do_include): Use INCLUDE_LEN_FUDGE.

From-SVN: r1488