Why is my variable definition not visible in my target_link_libraries usage site at a higher directory scope than the one the variable was defined in?

I am having issue with correctly linking/including a library in my c++ project. I have followed other posts and used add_subdirectory, target_include_directories, and target_link_libraries in CMakeLists.txt.
But I am still getting the following error during make.

test_app/main.cc:3:10: fatal error: icalendar.h: No such file or directory
 #include "icalendar.h"
          ^~~~~~~~~~~~~
compilation terminated.
CMakeFiles/program.dir/build.make:75: recipe for target 'CMakeFiles/program.dir/main.cc.o' failed
make[2]: *** [CMakeFiles/program.dir/main.cc.o] Error 1
CMakeFiles/Makefile2:117: recipe for target 'CMakeFiles/program.dir/all' failed
make[1]: *** [CMakeFiles/program.dir/all] Error 2
Makefile:90: recipe for target 'all' failed
make: *** [all] Error 2

The project structure looks like this:

.
├── CMakeLists.txt
├── icalendarlib
│   ├── CMakeLists.txt
│   ├── date.cpp
│   ├── date.h
│   ├── icalendar.cpp
│   ├── icalendar.h
│   ├── README.rst
│   ├── test
│   │   ├── calendar.ics
│   │   ├── CMakeLists.txt
│   │   └── iCalendarTest.cpp
│   ├── types.cpp
│   ├── types.h
│   └── wxwrapper.h
└── main.cc

The project’s CMake looks like this:

#./CMakeLists.txt
cmake_minimum_required(VERSION 3.6)
project(test_app)

add_subdirectory(icalendarlib)

add_executable (program main.cc)

target_link_libraries(program ${LIB_OUTPUT_NAME})

The library’s cmake looks like this:

#./icalendarlib/CMakeLists.txt
project(iCalendarlib)
set(LIB_OUTPUT_NAME iCalendar)

set(CFLAGS "-g")
SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${CFLAGS}")
SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${CFLAGS}")

set(CMAKE_CXX_STANDARD 11)
set(CMAKE_CXX_STANDARD_REQUIRED True)

add_library(${LIB_OUTPUT_NAME} SHARED
    date.cpp
    date.h
    icalendar.cpp
    icalendar.h
    types.cpp
    types.h
)

target_include_directories(${LIB_OUTPUT_NAME} PUBLIC ${CMAKE_SOURCE_DIR}/icalendarlib)

main.cc looks like this

#include "icalendar.h" // << fatal error: icalendar.h: No such file or directory
int main () {
  return 0;
}

I am running these commands under build folder

cmake .. 
make

>Solution :

There’s a rule I try to follow in CMake: don’t use variables if you don’t really have to.

For example, you could just link to the library directly instead of using a variable equals to the name of your library:

target_link_libraries(program iCalendar)

But here’s the problem: variables in CMake are scoped by directory. When you do this

target_link_libraries(program ${LIB_OUTPUT_NAME})

You’re not linking to any library since you set the value in the subdirectory only.