Fix configure's search for collateindex.pl.

PGAC_PATH_COLLATEINDEX supposed that it could use AC_PATH_PROGS to search
for collateindex.pl, but that macro will only accept files that are marked
executable, and at least some DocBook installations don't mark the script
executable (a case the docs Makefile was already prepared for).  Accept the
script if it's present and readable in $DOCBOOKSTYLE/bin, and otherwise
search the PATH as before.

Having fixed that up, we don't need the fallback case that was in the docs
Makefile, and instead can throw an understandable error if configure didn't
find the script.  Per recent trouble report from John Lumby.