I figured out, what the problem is: In the mkspec file qmake.conf the compiler flag Zc:wchar_t-
causes the compiler to translate some types relatet to wchar_t
incorrectly and hence the resulting symbols could not be resolved.
To solve this problem, change the above .pro file to
TEMPLATE = app
CONFIG += console
CONFIG -= app_bundle
CONFIG -= qt
QMAKE_CXXFLAGS += -EHsc -Zc:wchar_t
SOURCES += main.cpp
(add -Zc:wchar_t to QMAKE_CXXFLAGS).