Page labels: regression in 2.17

[Nicolas Sceaux]

Hi,

When a page label is refered to inside some music, the corresponding page is not
found. The delayed evaluation happens before the label pages are collected.

Compare:
   
http://lilypond.org/doc/v2.17/input/regression/collated-files.html#page-label.ly
which reads "A (page ?)", with:
   
http://lilypond.org/doc/v2.16/input/regression/collated-files.html#page-label.ly
which reads "A (page 3)"

Nicolas