CMake: Build and install locally stored submodule - c++

Question

There are numerous similar questions on Stack Overflow, but none come near to answering my question.
I have a C++ library built using CMake:
mylib
| - CMakeLists.txt
| - src/
| | - m.h
| | - m.cpp
| - include/
| | - mylib/
| | | - a.h
| | | - something/
| | | | - some.h
| - cmake/
| - mylibConfig.cmake.in
| - mylibConfigVersion.cmake.in
I then create another library or executable which includes the aforementioned library:
myapp
| - CMakeLists.txt
| - src/
| | - main.cpp
| - include/
| | - myapp/
| | | - b.h
| - libs
| | - mylib
And would like to use mylib within myapp like so. Please take notice how mylib headers are included in a directory like format:
#include <mylib/a.h>
#include <mylib/something/some.h>
mylib should be built when building myapp so that the following works without any other build steps:
$ cd myapp/build
$ cmake ..
$ make
Here is a list of some of the Stack Overflow posts I have reviewed. I have attempted each, and they simply do not work:
CMake: How to build external projects and include their targets
CMake ExternalProject_Add() and FindPackage()

Answer 1

Assuming in the project described, only header files in the mylib/include directory are to be considered public:
The top-level CMakeLists.txt for mylib needs to contain:
cmake_minimum_required(VERSION 3.13 FATAL_ERROR)
project(mylib VERSION 0.1 LANGUAGES CXX)
add_library(${MyLibName} ${MyLib_SOURCES})
target_include_directories(${MyLibName}
PUBLIC
$<INSTALL_INTERFACE:include>
$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
PRIVATE
${CMAKE_CURRENT_SOURCE_DIR}/src
)
This ensures that projects including mylib will only have access to files in the include/ directory. Files in include/ can be used like #include <myfile.h> and files in include/mylib (the general convention) can be used like #include <mylib/myfile.h>.
The top-level CMakeLists.txt for myapp should include:
cmake_minimum_required(VERSION 3.13 FATAL_ERROR)
project(myapp VERSION 0.1 LANGUAGES CXX)
add_subdirectory(libs/mylib)
add_executable(${MyAppName} ${MyApp_SOURCES})
target_link_libraries(${MyAppName} ${MyLibName}
The use of add_subdirectory ensures that mylib will be built before myapp and target_link_libraries adds mylib to the executable.
As mentioned by Tzalumen, make sure you take a look at the CMake tutorials and prefer the use of cmake --build . instead of make.

Answer 2

So, I actually do this in one of my projects.
First, the library needs to be built with ADD_LIBRARY(), but I'm assuming you do that. Something like
ADD_LIBRARY (${MyLibName} ${SOURCES}) in the library's CMakeLists.txt
That adds the library into cmake's list of libraries.
Second, you add the library to your project with
target_link_libraries (${PROJECT_NAME} ${MyLibName}) in the executable's CMakeLists.txt, the same one where you have your add_executable (${PROJECT_NAME} ${SOURCES})
That will set up the dependency chain to force MyLibName to explicitly build before PROJECT_NAME.