Bug 90255

  Conflict about "index.sgml" may cause index unable to be correctly built.
Links in index in single HTML are pointed to HTML-split files *.html.
You can have a try:
ftp://ftp.freebsd.org/pub/FreeBSD/doc/en_US.ISO8859-1/books/arch-handbook/book.html.tar.bz2

  And I cannot build ${DOC}.rtf alone.

  What's in my patch:

1.Regard "index.sgml" as temporary file, no longer as what ${DOC}.* depend on.
  And now generate "index.sgml" when updating targets ${DOC}.* .
  This measure solves the conflict that all targets depend on deferent but
  commonly-named "index.sgml"s.

  Related targets:

    index.html HTML.manifest
    ${DOC}.html
    ${DOC}.html-text
    ${DOC}.rtf
    ${DOC}.tex
    ${DOC}.tex-pdf

2.Let ${DOC}.rtf depends on ${PRINT_INDEX} and ${LOCAL_IMAGES_PNG} .
  Otherwise, it will depend on ${DOC}.html to generate ${PRINT_INDEX} and
  ${LOCAL_IMAGES_PNG}.

3.Delete target ${INDEX_SGML}, which was used to initialize index.sgml.

4.Modification on targets ${HTML_INDEX} and ${HTML_SPLIT_INDEX}:
  a. Let them depend on ${SRCS} ${LOCAL_IMAGES_TXT} .
  b. Initialize index.sgml by themselves.
  c. No longer generate index.sgml here, which is generated by ${DOC}.* .