How to write CMakeLists.txt correctly?

Question

C++ / C#

Good afternoon, there was a problem in writing a file for cmake. I tried to solve it myself, but in the lessons, as a rule, most of the examples use standard situations.
In particular, I have this problem.
Here is the project structure:
src/
main.cpp
class1.cpp
class2.cpp
class3.cpp
include/
class1.h
class2.h
class3.h
Plus there are three more header files that are used by other libraries. All of them are in the system folder in /usr/include in ubuntu. They also use two keys. Actually, the difficulty lies in the fact that, as a rule, these are not some kind of modular libraries that are in folders, but simply header files. Moreover, I would like to make it so that I do not explicitly specify the path, but cmake itself looked for them.
I would like to see an example of how it should look or at least point in the right direction.
Thanks in advance!

Answer 1

First you look for the CMake library (find_package/pkg_check_modules/find_path), they set the CACHE defined variables that you use in your target properties (target_link_libaries). If the external library does not support searching through CMake, then they usually write and include the FindSomeLib.cmake file, where they call the basic search utilities (like find_path) and create an IMPORTED target containing all the necessary flags for compilation.

Answer 2

If this is the entire problem area, then most likely the last dot should be a comma ...