Error when opening "src.pro" in SDK

[Asperamanca]

I am using the SDK, with Qt Creator on MinGW

When I try to open the Source project "src.pro" in either my 4.7.3 or 4.7.4 source tree, I get a ton of error messages that sub-projects cannot be opened:

bq. Could not find .pro file for sub dir 'src_tools_bootstrap' in '/src/tools/bootstrap'

and so on, for pretty much every sub-project.
The only subproject I see in the project tree is tools.pro.

I know this used to work on my installation, so I suspected a botched update.
How can I fix my src.pro file?

I noticed the pro-file makes extensive use of the variable $$QT_SOURCE_TREE.

So I tried to set QT_SOURCE_TREE in my Qt project settings, but that didn't work.