Problem building doc section without built LilyPond

[Urs Liska]

When trying to build a documentation section with 
scripts/auxiliar/doc-section.sh I encounter a problem for which I can't 
provide a patch myself:

The path to lilypond-book is defined as
LILYPOND_BOOK="$LILYPOND_BUILD_DIR/out/bin/lilypond-book"
in line 77.

But if I haven't built LilyPond myself, there isn't anything present of 
course.

If I change that line to point to the lilypond-book of my regular 
install the script works.

Obviously the line should first look in that place but somehow figure 
out the/a LilyPond installation by itself if it doesn't find one.

Best
Urs