The problem is caused by revision 578, i.e. the Qt5 upgrade:
wva@yup:~/splintermind-attributes$ hg up -C 577
128 files updated, 0 files merged, 21 files removed, 0 files unresolved
wva@yup:~/splintermind-attributes$ qmake
Project MESSAGE: Release Mode
Project MESSAGE: Setting up for Linux
Project MESSAGE: Setting up for Linux Install
wva@yup:~/splintermind-attributes$ hg up -C 578
129 files updated, 0 files merged, 0 files removed, 0 files unresolved
wva@yup:~/splintermind-attributes$ qmake
Project MESSAGE: Release Mode
Project MESSAGE: Setting up for Linux
Project MESSAGE: Setting up for Linux Install
Project MESSAGE: Warning: unknown QT: widgets
wva@yup:~/splintermind-attributes$ hg log -r 578
changeset: 578:4294379f597b
user: joshb
date: Thu Oct 03 17:39:35 2013 +0200
summary: v.20.6 ** updated the project to qt 5.1.1 **
I did get a bit further by removing qt4 and installing qt5 and some additional scripts:
$ sudo apt-get remove --purge qt4-default qt4-designer qt4-dev-tools qt4-qmake qt4-doc qt4-linguist-tools qtchooser
[..]
$ sudo apt-get install qt5-qmake qt5-default qttools5-dev-tools libqt5script5 libqt5scripttools5 qtscript5-dev
[..]
$ qmake
Project MESSAGE: Release Mode
Project MESSAGE: Setting up for Linux
Project MESSAGE: Setting up for Linux Install
$
Edit: Never mind, I'm being stupid, I never undid the 'hg up -C 578' earlier, so I was building an old version. Will retry with the newest version and post results.
Edit: Victory! The problem was caused by building an old version, stupid me. So, if you uninstall and install the packages mentioned above, remove 'bin' if it exists, and qmake+make, it works (at least on my ubuntu 13.10).
(probably you can make do with less packages, I simply added stuff until it stopped complaining...)