project/
  CMakeLists.txt
  subprojectA/
    CMakeLists.txt
    include/
      headerA.hpp
    src/
      libraryA.cpp
  subprojectB/
    CMakeLists.txt
    src/
      mainB.cpp

project(SubProjectA)
include_directories(include)
add_library(subprojectA STATIC src/libraryA.cpp)
set(${PROJECT_NAME}_INCLUDE_DIRS
    ${PROJECT_SOURCE_DIR}/include
    CACHE INTERNAL "${PROJECT_NAME}: Include Directories" FORCE)

project(SubProjectB)
include_directories(${SubProjectA_INCLUDE_DIRS})
add_executable(mainBinary src/mainB.cpp)
target_link_libraries(mainBinary subprojectA)

project(Project)
add_subdirectory(subprojectB)
add_subdirectory(subprojectA)

subprojectB/src/mainB.cpp:1:23: fatal error: headerA.hpp: No such file or directory
#include "headerA.hpp"
                      ^