[Boost-build] Patch for property.jam to skip rules that are already in indirect format

From: Roland Schwarz (roland.schwarz_at_[hidden])
Date: 2007-10-29 04:08:57

Attached is a patch for property.jam that will not translate "@" rules 
that are already in indirect format.

This is e.g. necessary when one wants to remove a property with "-" that
has a rule attached.

E.g. -<tag>$(module)%$(module).tag
will remove the tag rule that has been defined in $(module).

Volodya: could you please look if this patch passes your test suite?

Roland aka speedsnail