doco header index entries

[Kevin Ryde]

I noticed the lack of an entry for stdint.h, when trying to hit the
"Portability of Headers" node.

        * autoconf.texi: Add index entries for various standard header files.

Is @cindex right place for such things?  It's got acconfig.h so
perhaps is close enough.

These additions are confined to places where something specific is
said about the header file availability or contents.  So for instance
the note about sys/ucred.h has an index entry for that file, but not
for sys/types.h which it requires.

I deliberately omitted entries for the obsolete macros, so as not to
have an "i" index entry lookup hit them.

autoconf.texi.header-indexing.diff
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