/MainProject/inc/main.h
/MainProject/src/main.cpp
/LibProject/inc/test.h
/LibProject/src/test.cpp

root
|-MainProject
| |-inc
| | '-main.h
| |-src
| | |-main.cpp
| | '-CMakeLists.txt
| '-CMakeLists.txt
|-LibProject
| |-inc
| | '-test.h
| |-src
| | |-test.cpp
| | '-CMakeLists.txt
| '-CMakeLists.txt
'-CMakeLists.txt

project(MyProject)
subdirs(MainProject LibProject)

subdirs(src)

# Notice name prefix of this variable, set by CMake according
# to value given with "project()" in the root CMakeLists.txt.
include_directories(${MyProject_SOURCE_DIR}/LibProject/inc)
add_library(LibProject test.cpp)

include_directories(${MyProject_SOURCE_DIR}/MainProject/inc)
# I assume you want to use LibProject as a library in MainProject.
include_directories(${MyProject_SOURCE_DIR}/LibProject/inc)
link_directories(${MyProject_SOURCE_DIR}/LibProject/src)
add_executable(MainProject main.cpp)
target_link_libraries(MainProject LibProject)

$ cd root
$ mkdir build
$ cd build
$ cmake ..
$ make

project(MainProject)

add_subdirectory(LibProject/src)
add_subdirectory(MainProject/src)

include_directories(${PROJECT_SRC_DIR}/LibProject/inc/)
add_library(LibProject test.cpp)

include_directories(${PROJECT_SRC_DIR}/MainProject/inc/)
add_executable(MainProject main.cpp)
target_link_libraries(MainProject LibProject)

My structure:
Project
|CMakeLists.txt
|-src
| |*.cpp
| |*.c
|-include
| |*.hpp
| |*.h

include_directories(
        ${PROJECT_SOURCE_DIR}/include
        ${PROJECT_SOURCE_DIR}/src
)

file(GLOB all_SRCS
        "${PROJECT_SOURCE_DIR}/include/*.h"
        "${PROJECT_SOURCE_DIR}/include/.hpp"
        "${PROJECT_SOURCE_DIR}/src/*.cpp"
        "${PROJECT_SOURCE_DIR}/src/*.c"
        )