Rework our index support.

• Add a new document-specific variable, HAS_INDEX, to specify if a given document is marked up with <indexterm> entries.
• Rework the index support so that both HAS_INDEX and GEN_INDEX are checked before trying to generate an index for a document.
• Only create index.sgml if both HAS_INDEX and GEN_INDEX are set.

This allows us to recursively build the documentation tree with
GEN_INDEX=1 and have it only try to create an index (very time
consuming) for the few documents that are ready for this. Previously,
running "make GEN_INDEX=1" from the top of the doc tree would look for
index terms in every single document.

With this, I hope we can start building our docs with GEN_INDEX set on
freefall so that users browsing the HTML docs will get the benefit of
the index we've been hiding in CVS for 6 months.