cpplib.h (struct cpp_options): Add canonical_system_headers.

    * include/cpplib.h (struct cpp_options): Add canonical_system_headers.
    * files.c (find_file_in_dir): Call maybe_shorter_path() only if
    canonical_system_headers is set.
    * init.c (cpp_create_reader): Initialize canonical_system_headers.
    * configure.ac: Add new --enable-canonical-system-headers.
    * configure: Regenerate.
    * config.in: Regenerate.

    * doc/cppopts.texi: Document -f[no-]canonical-system-headers.
    * doc/install.texi: Document --enable-canonical-system-headers.

    * c.opt: Add f[no-]canonical-system-headers.
    * c-opts.c (c_common_handle_option): Handle
    OPT_fcanonical_system_headers.

From-SVN: r193569