Qt 5.5 with qmake: Linker cannot resolve OpenGL function calls

When using Qt 5.5, qmake and MSVC 13 to compile a basic, boilerplate Qt application with some fundamental OpenGL function calls, I get the following linker errors:

glwidget.obj:-1: error: LNK2019: unresolved external symbol __imp__glClear@4 referenced in function "public: virtual void __thiscall GLWidget::initializeGL(void)" (?initializeGL@GLWidget@@UAEXXZ)
glwidget.obj:-1: error: LNK2019: unresolved external symbol __imp__glClearColor@16 referenced in function "public: virtual void __thiscall GLWidget::initializeGL(void)" (?initializeGL@GLWidget@@UAEXXZ)
debug\OpenGLApp.exe:-1: error: LNK1120: 2 unresolved externals

I have:

Specified QT += opengl
Explicitly specified CONFIG += windows (apparently += console disables gui features)

the .pro file:

QT       += core gui opengl widgets

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets opengl

TARGET = OpenGLApp
TEMPLATE = app

CONFIG += windows

SOURCES += main.cpp\
        mainwindow.cpp \
    glwidget.cpp

HEADERS  += mainwindow.h \
    glwidget.h

the glwidget.cpp file:

#include "glwidget.h"

GLWidget::GLWidget(QWidget *parent) : QOpenGLWidget(parent) {

}

void GLWidget::initializeGL() {
    glClearColor(0.0f, 0.0f, 1.0f, 1.0f);
    glClear(GL_COLOR_BUFFER_BIT);
}

glwidget.h file:

#include <QOpenGLWidget>
#include <QOpenGLFunctions>

class GLWidget : public QOpenGLWidget {
    Q_OBJECT
public:    
    GLWidget(QWidget *);

    void initializeGL();
    void resizeGL();
    void PaintGL();

};

In another virtually identical test program, I had the same problem of the linker being unable to resolve OpenGL function calls. By using CMake instead, specifically with the following "find_package(OpenGL REQUIRED)" line, and the addition of "${OPENGL_LIBRARIES}" in "target_link_libraries" I was able to solve the problem:

#Qt5
find_package(Qt5Core REQUIRED)

find_package(Qt5Widgets REQUIRED)
find_package(Qt5Gui REQUIRED)
find_package(Qt5OpenGL REQUIRED)

#OpenGL
find_package(OpenGL REQUIRED)

target_link_libraries(${PROJECT_NAME} Qt5::Widgets Qt5::Gui Qt5::Core Qt5::OpenGL ${OPENGL_LIBRARIES})

I therefore suspect qmake is unable to find the OpenGL libraries, although I am unsure as how to check and what may be the cause of this, and so would appreciate if someone could point out to me what I'm missing.

You need to add in .pro file