Getting a CMake Error: Cannot specify link libraries for target which is not built by the project

Question

I am implementing CMake in my code but I'm getting the error

"Cannot specify link libraries for target "Qt5::Widgets" which is not built by the project".

Below are the contents of the CMakeLists.txt:

#Specify the version being used aswell as the language
cmake_minimum_required(VERSION 2.6)

#Name your project here
project(eCAD)

#Sends the -std=c++11 flag to the gcc compiler
ADD_DEFINITIONS(-std=c++11)

#This tells CMake to main.cpp and name it eCAD
add_executable(eCAD main.cpp)


#include the subdirectory containing our libs
add_subdirectory (gui)
include_directories(gui)

#include Qt directories
find_package(Qt5Widgets)
find_package(Qt5Core)
find_package(Qt5Designer)
SET(QT_USE_QTDESIGNER ON)

#link_libraries
target_link_libraries(Qt5::Widgets Qt5::Core) 

Answer 1

In addition to the accepted answer: An important detail is to place target_link_libraries after the add_executable and find_package lines, so all linked components are known.