[boost] Invoking a library's build//stage ignores --stagedir?

Subject: [boost] Invoking a library's build//stage ignores --stagedir?
From: Peter Dimov (lists_at_[hidden])
Date: 2016-04-14 11:59:04

Let's suppose that for some random reason I want to stage Boost.System into 
c:\stage with the current directory being outside the Boost root, which is 
at c:\projects\boost-git\boost:

C:\Users\Peter 
Dimov>b2 -sBOOST_ROOT=c:\projects\boost-git\boost --stagedir=c:\stage 
c:/projects/boost-git/boost/libs/system/build//stage

This doesn't work. Or rather, it works, but stages into 
C:\projects\boost-git\boost\libs\system\stage, ignoring the --stagedir.

I have no idea why. I tried to debug with -d+7, and BOOST_STAGE_LOCATE is 
set correctly from --stagedir, but then Boost.System is staged to its local 
stage directory.

Does anyone have an idea what's wrong here?