Add the index entity. This entity points to index.sgml which will be
created automatically by the makefiles either with collageindex.pl or
with the touch command, depending on the value of GEN_INDEX.

To build the handbook with an index simply type :

make GEN_INDEX=1

or for print output :

make GEN_INDEX=1 FORMATS=ps

by default, the index is turned off since it dramatically increases
the time required to build the handbook.
PH: