Using the Taglib library in a CMake project while keeping it cross platform

Question

I am trying to use CMake for the first time by making a cross-plaform project (Windows and Linux, but not macOS) with the Taglib library (https://github.com/taglib/taglib).

My problem right now is that I receive the "No such file or directory" error when I build the application using the "make" software. This is because the path to the Taglib header files are not specified to the compiler by the -I option. How can I use the information in the CMake files in the Taglib project to compile with its headers (with the -I option).

Plus, if some of you can give me tips on how to add the Taglib library in a CMake projet in order to make it cross platform it would be much appreciated.

Thanks again, and I mean it a lot!

Project Tree Structure

    ProjectName/
        |-- Preload.cmake
        |-- CMakeLists.txt
        |-- main.cpp
        |-- taglib/ (This folder is a submodule to my repository. This repository can be found here: 
                     https://github.com/taglib/taglib)

ProjectName/Preload.cmake

    if(UNIX AND NOT APPLE)
        set (CMAKE_GENERATOR "Unix Makefiles" CACHE INTERNAL "" FORCE)
    endif()
    if(WIN32)
        set (CMAKE_GENERATOR "MinGW Makefiles" CACHE INTERNAL "" FORCE)
    endif()

ProjectName/CMakeLists.txt

    cmake_minimum_required(VERSION 3.10)
    project(ProjectName)
    set(CMAKE_CXX_STANDARD 17)
    set(CMAKE_CXX_STANDARD_REQUIRED True)
    
    add_subdirectory(taglib)
        
    # Here I should include the directories of Taglib.
    
    # I am not sure aobut this but I am guessing that it is here that I should
    # set some CMake configuration variable or some C++ Pre-compiler definition.
    # Here is the installation note (https://github.com/taglib/taglib/blob/master/INSTALL.md).

    add_executable(${CMAKE_PROJECT_NAME} main.cpp)

    target_link_libraries(${CMAKE_PROJECT_NAME} tag)

ProjectName/main.cpp

    // Some Taglib included headers
    #include <mpegfile.h>
    #include ...
    #include ...

    int main(int argc, char* argv[]) {
        // Some code using the taglib library
    }