Implementing Qt project through CMake

Your script has several errors, also a few things can be improved. After changes it will be looks like:

cmake_minimum_required(VERSION 3.0.2)
project(MyProject)

find_package(Qt5Widgets)

set(CMAKE_AUTOMOC ON)
set(CMAKE_AUTOUIC ON)
set(CMAKE_INCLUDE_CURRENT_DIR ON)

add_library(mainwindow mainwindow.cpp)
target_link_libraries (mainwindow Qt5::Widgets)

add_executable(MyProject main.cpp)
target_link_libraries (MyProject mainwindow)

Errors:

1. Wrong add_executable directive. You try to add a library, but for this purpose you need to use target_link_libraries. So instead of:

   add_executable(Cmake main.cpp mainwindow)
   

   You need:

   add_executable(Cmake main.cpp)
   target_link_libraries (Cmake mainwindow)
   

2. And one more mistake is missing *.cpp files in the add_library directive:

   add_library(mainwindow mainwindow.cpp ${Cmake_hdr_moc} ${Cmake_form_hdr})

Recommendations:

1. Also setting version of CMake would be appropriate. If you use CMAKE_AUTOMOC you need a version not less than 2.8.6, and if you use CMAKE_AUTOUIC you need a version not less than 3.0.2:

   cmake_minimum_required(VERSION 3.0.2)
   

2. Using qt5_wrap_cpp with CMAKE_AUTOMOC isn't necessary.

3. When you use CMAKE_AUTOMOC usage CMAKE_AUTOUIC instead of qt5_wrap_ui will be more appropriate.

4. This script is correct for the project with the following structure in the file system:

   Project
   ├── CMakeLists.txt
   ├── main.cpp
   ├── mainwindow.cpp
   ├── mainwindow.h
   └── mainwindow.ui
   

   If you have another structure you should use include_directories as was mentioned by @steveire.

5. (UPD) Since, I've written this answer, I suggested it several times for beginners who try to meet with Qt through CMake. They complain of an inappropriate name of the project - "Cmake". For beginners who just meet with CMake is difficult to realize where cmake - is just part of the project name (and isn't mandatory) and where cmake is part of a directive (and is mandatory). So I'd like to replace the name of the project from "Cmake" to "MyProject". This reduces connection between question and answer, but on the other hand this makes the answer more friendly for beginners.

6. (UPD) As was mentioned by @Erik Sjölund qt5_use_modules is obsolete and target_link_libraries should be used instead.

Note: Personally I have had unsuccessful experience with CMAKE_AUTOMOC; it's good for a simple project with plain structure. I've had problems with a case when my include files were stored into a separate directory:

.
├── include
│   └── QtClass.h
└── src
    └── QtClass.cpp

And when files with the same name were into different subdirectories:

.
├── NamespaceA
│   ├── QtClass.cpp
│   └── QtClass.h
└── NamespaceB
    ├── QtClass.cpp
    └── QtClass.h

Finally: This is a suggestion based on my personal opinion, but I'd like to propose more explicit version of the script without usage of CMAKE_AUTOMOC and CMAKE_AUTOUIC, it's more verbose but in other hand you have more control:

cmake_minimum_required (VERSION 2.8.12)
project (MyProject)

find_package (Qt5Widgets)

set (MyProjectLib_src ${PROJECT_SOURCE_DIR}/mainwindow.cpp)
set (MyProjectLib_hdr ${PROJECT_SOURCE_DIR}/mainwindow.h)
set (MyProjectLib_ui  ${PROJECT_SOURCE_DIR}/mainwindow.ui)
set (MyProjectBin_src ${PROJECT_SOURCE_DIR}/main.cpp)

qt5_wrap_cpp(MyProjectLib_hdr_moc ${MyProjectLib_hdr})
qt5_wrap_ui (MyProjectLib_ui_moc  ${MyProjectLib_ui})

include_directories (${PROJECT_SOURCE_DIR})
include_directories (${PROJECT_BINARY_DIR})

add_library (MyProjectLib SHARED 
    ${MyProjectLib_src}
    ${MyProjectLib_hdr_moc}
    ${MyProjectLib_ui_moc}
)
target_link_libraries (MyProjectLib Qt5::Widgets)

add_executable(MyProject ${MyProjectBin_src})
target_link_libraries (MyProject MyProjectLib)

The complete version of the projects source code is available at GitLab.