Do not inherit standlone project from user-config.
Since standalone projects can be defined by toolset modules, invoked
from site-config.jam and user-config.jam, inheriting from not yet fully read
modules is tricky. Especially when toolset module is invoked from site-config.
We can't inherit from user-config, if we did not even start to load it!
[SVN r28212]
Vladimir Prus