How to use modern cmake for this project - c++

Question

I have for some time had some problems incorporating modern cmake into a project I've been working fr some while and seem to be stuck on how i should define the the individual CMakeLists, and the top-level CMakeList.
My project structure is pretty simple.
├── build
└── src
├── include
│   ├── database
│   │   ├── database.cpp
│   │   └── database.h
│   ├── match
│   │   ├── match.h
│   │   └── mathc.cpp
│   ├── record
│   │   ├── lib
│   │   ├── record.cpp
│   │   └── record.h
│   └── spectogram
│   ├── spectogram.cpp
│   └── spectrogram.h
└── main.cpp
main.cpp are linked to all the includes, and some of the includes should know the presence of other includes, meaning, I should be able to include match.h in
database.h. Some third-party libs are also going to be used, in this case I am using portaudio, download and installed using externalproject_add, which should only be visible for the include which holds the library, in this case record, should only see this.
But how I should define the individual CMakeList, is currently unknown.
I've scouted the net for a proper way of setting this up, but cannot seem to find one that I understand.
How do i define the CMakeLists for this project, and how do i make sure that the includes, are visible for the Main.cpp and the includes files that need them, and how do I make third-party-libraries visible only for the includes that it is being used for.
CMakeLists example structure tried:
CMakeLists.txt
cmake_minimum_required(VERSION 3.2)
project(soundcloud)
#add_subdirectory(src/database)
#add_subdirectory(src/match)
#add_subdirectory(src/record)
add_subdirectory(src/include/spectogram)
add_executable(cmakeDemo src/main.cpp)
SET_TARGET_PROPERTIES(cmakeDemo PROPERTIES LINKER_LANGUAGE Cxx)
target_link_libraries(cmakeDemo spectogram)
#target_link_libraries(cmakeDemo database match record spectogram)
src/include/database/CMakeLists.txt
add_library(spectogram STATIC .)
target_include_directories(spectogram PUBLIC .)
getting error message:
CMake Error: Error required internal CMake variable not set, cmake may not be built correctly.
Missing variable is:
CMAKE_Cxx_LINK_EXECUTABLE
CMake Error: Cannot determine link language for target "spectogram".
CMake Error: CMake can not determine linker language for target: spectogram

Answer 1

You can use the PRIVATE property of target_include_directories. When PRIVATE, it means that the include directories will be available to the sources of the target. When PUBLIC, it will also be available to whomever links to the target.
If I were writing this in a single CMakeLists.txt, I'd do this:
cmake_minimum_required(VERSION 3.0)
add_library(database STATIC src/include/database.cpp)
target_include_directories(database PUBLIC src/include/database)
################
add_library(match STATIC src/include/mathc.cpp)
target_include_directories(match PUBLIC src/include/match)
################
include(ExternalProject)
ExternalProject_Add(portAudio ${SomeCommands})
add_library(record STATIC src/include/record.cpp)
target_include_directories(record PUBLIC src/include/record) # When cmakeDemo links to this, it'll get these includes
target_include_directories(record PRIVATE src/include/record/lib) # When cmakeDemo links to this, it won't get these includes
target_link_libraries(record portAudio)
#################
add_library(spectogram STATIC src/include/spectogram.cpp)
target_include_directories(spectogram PUBLIC src/include/spectogram)
##################
add_executable(cmakeDemo src/main.cpp)
target_link_libraries(cmakeDemo database match record spectogram)
If I were to do this with distributed CMakeLists.txt, I'd split the files where the ###### lines are, fix the paths, and use add_subdirectory() to include the sub-directories from higher-level cmake files.

Answer 2

First, you don't link to includes, you just "include" them during compilation.
Some people put one CMakeList.txt into every directory that contains a fairly independent compilation unit.
Some people just use one big one at the the top.
Having just one CMakeList.txt file makes it easier to start, but if the project gets huge, things get messy.
For every compilation unit, you can specify the include directories with target_include_directories