Glyph of Destruction was originally worded: "Target wall you control gains +10/+0 when blocking." To me, that indicates that you could cast it any time, i.e. outside of combat.
However, the Gather wording is:
Target blocking Wall you control gets +10/+0 until end of combat. Prevent all damage that would be dealt to it this turn. Destroy it at the beginning of the next end step.
Since Gatherer now says "Target blocking Wall", does that mean I have to declare the Wall as a blocker before I cast the Glyph? I'm trying to come up with some Glyph/Fling shenanigans for a deck and I need to know if I have to block before I use this trick.