[Boost-build] Non-buildable targets.

From: Rene Rivera (grafikrobot_at_[hidden])
Date: 2007-08-28 11:01:01

Not that I have time to look into how to do this right now, but I 
thought I'd bring it up for others to think about...

In testing I'm seeing some testers get this set of errors:

   ...
   warning: Python location is not configured
   warning: the Boost.Python library won't be built
   ...
   error: Unable to find file or target named
   error:     '/boost/python//boost_python'
   error: referred from project at
   error:     '../libs/parameter/test'

The reason for the error is obvious when looking at 
libs/python/build/Jamfile.v2:

   ...
   if [ python.configured ] {
     ...
     lib boost_python ...
   }
   else {
     ECHO "warning: ...
   }

Instead of not declaring the target it would be better to declare an 
alternate target that will:

   * Fail unconditionally when it tries to build.
   * Gives some suitable message when it fails to build.
   * Has the effect of only adding a <dependency> to the dependents.

Perhaps something declared as:

   unavailable boost_python :
     "warning: Python location is not configured"
     "warning: the Boost.Python library won't be built"
     ;

This way dependent targets just fail to build instead of causing errors 
that interrupt the overall build. Which in the testing case causes the 
rather more appealing error boxes as opposed to no results.

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