Windows: not possible to compile Qt when sources installed from the installer

Download Qt5.1.1 opensource offline installer (tested with MSVC2010 angle installer).
Install Qt with sources only.
Go to $QTDIR\Src\qtbase and run configure. Following options used while testing:
-opensource -developer-build -confirm-license -nomake tests -nomake examples
Set Qt into the path and run jom (or nmake).

Bug:
Error while compiling Qt:

win_flex --noline --nounistd --outfile=Tokenizer.cpp ..\..\..\..\3rdparty\angle\src\compiler\preprocessor\Tokenizer.l
'win_flex' is not recognized as an internal or external command,
operable program or batch file.
NMAKE : fatal error U1077: 'win_flex' : return code '0x1'

--> bad user experience; it should be possible to compile Qt sources installed via offline installer.