Qt5 cmake Windows Linker Error - c++

Question

I just wanted to start a new project which uses Qt in combination with CMake. I (hope I) followed the docs, but it seems as if the linker is not doing its work correctly:
CMakeFiles\QtTest.dir/objects.a(main.cpp.obj): In function `QString::QString(char const*)':
P:/Qt/5.9.1/msvc2015_64/include/QtCore/qstring.h:659: undefined reference to `__imp__ZN7QString16fromAscii_helperEPKci'
CMakeFiles\QtTest.dir/objects.a(main.cpp.obj): In function `QTypedArrayData<unsigned short>::deallocate(QArrayData*)':
P:/Qt/5.9.1/msvc2015_64/include/QtCore/qarraydata.h:237: undefined reference to `__imp__ZN10QArrayData10deallocateEPS_yy'
I'm using mingw-w64 (the x86_64 installation) with CLion and its bundled CMake and Qt 5.9.1. My CMakeLists.txt looks like this:
cmake_minimum_required(VERSION 3.8)
project(QtTest)
set(CMAKE_CXX_STANDARD 14)
set(CMAKE_MODULE_PATH ${PROJECT_SOURCE_DIR})
set(CMAKE_VERBOSE_MAKEFILE ON)
# Qt
set(CMAKE_INCLUDE_CURRENT_DIR ON)
set(CMAKE_AUTOMOC ON)
set(Qt5_NO_LINK_QTMAIN ON)
set(CMAKE_PREFIX_PATH P:/Qt/5.9.1/msvc2015_64/lib/cmake)
find_package(Qt5Core REQUIRED)
get_target_property(QtCore_location Qt5::Core LOCATION)
message("QtCore version ${Qt5Core_VERSION} is at ${QtCore_location}")
# QtTest executable
set(SOURCE_FILES main.cpp)
add_executable(QtTest ${SOURCE_FILES})
target_link_libraries(QtTest Qt5::Core)
When creating the build files it outputs (correctly) QtCore version 5.9.1 is at P:/Qt/5.9.1/msvc2015_64/bin/Qt5Core.dll
The main.cpp contains just a call to QString for testing purposes:
#include <QtCore/QString>
int main() {
QString string = QString("test");
}
Why does this happen?