1. CMakeLists.txt

cmake_minimum_required(VERSION 3.0)

project(test)
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fPIC -DPIC -Wno-deprecated")
set(CMAKE_AUTOMOC ON)

include_directories(${PROJECT_BINARY_DIR}/../include)
set(Qt5Core_DIR /usr/lib/x86_64-linux-gnu/qt5/lib/cmake/Qt5Core)
find_package(Qt5Core REQUIRED)
set(Qt5Widget_DIR /usr/lib/x86_64-linux-gnu/qt5/lib/cmake/Qt5Widgets_DIR)
find_package(Qt5Widgets REQUIRED)

add_library(mainwindow SHARED  lib/mainwindow.cpp)
add_executable(main main.cpp include/mainwindow.hpp)
target_link_libraries(main mainwindow ${Qt5Core_LIBRARIES} ${Qt5Widgets_LIBRARIES})

2. main.cpp

#include <QtWidgets/QApplication>
#include <mainwindow.hpp>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    return a.exec();
}

3. include/mainwindow.hpp

#ifndef __MAINWINDOW_HPP__
#define __MAINWINDOW_HPP__

#include <QtWidgets/QtWidgets>

class MainWindow: public QWidget
{
    Q_OBJECT

private:

public:
    explicit MainWindow(QWidget *parent = 0);

    ~MainWindow();

signals:

private slots:

};

#endif

4. lib/mainwindow.cpp

#include <mainwindow.hpp>

MainWindow::MainWindow(QWidget *parent) : QWidget(parent)
{

}

MainWindow::~MainWindow()
{

}