Improve the docbook build infrastructure's index.sgml build process:

• Consider index.sgml as intermediate file, not dependency that ${DOC}.* require. Instead, generate it on-the-fly when building ${DOC}.*. This solves the problem that index.sgml is generated for the first built target, but needs to be different from target to target.
  
  The affected targets are:

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

${INDEX_SGML} target removed in favor of the on-the-fly build.

• When building ${DOC}.rtf, add ${PRINT_INDEX} and ${LOCAL_IMAGES_PNG} as dependencies, eliminating the need of building ${DOC}.rtf without having to build ${DOC}.html beforehand.
• Modifications against ${HTML_INDEX} and ${HTML_SPLIT_INDEX}:

+ Add dependency to ${SRCS} and ${LOCAL_IMAGES_TXT} to force

	  index updates when necessary.

+ Every build now causes index.sgml to be reinitialized.
+ Depend on ${DOC}.* to generate index.sgml.

Submitted by: "intron" <intron at intron ac>
PR: docs/90255 (slightly changed version)