Best way to add include directories of root project to a subdirectory in CMake - cmake

Question

I have a project with a CMakeLists.txt at the root, which includes a project in a subdirectory test/ using add_subdirectory with the flag EXCLUDE_FROM_ALL. The tests need all of the include directories of the parent project. What would be the most elegant way to do this?
CMakeLists.txt
CMAKE_MINIMUM_REQUIRED(VERSION 3.1)
SET(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_SOURCE_DIR}/cmake/Modules")
PROJECT(autoffi CXX)
SET(CMAKE_CXX_STANDARD 11)
SET(AutoFFI_VERSION_MAJOR 0)
SET(AutoFFI_VERSION_MINOR 1)
SET(Boost_USE_STATIC_LIBS OFF)
FIND_PACKAGE(BOOST 1.50 COMPONENTS system filesystem REQUIRED)
FIND_PACKAGE(LLVM 3.9 REQUIRED CONFIG)
ADD_DEFINITIONS(${LLVM_DEFINITIONS})
FIND_PACKAGE(Clang REQUIRED)
CONFIGURE_FILE(
"${PROJECT_SOURCE_DIR}/include/env.h.in"
#"${PROJECT_BINARY_DIR}/include/env.h"
"${PROJECT_SOURCE_DIR}/include/env.h" # added to VCS
)
INCLUDE_DIRECTORIES(
#"${PROJECT_BINARY_DIR}"
"${PROJECT_SOURCE_DIR}/include"
"${CLANG_INCLUDE_DIRS}"
"${Boost_INCLUDE_DIRS}"
)
# The C++ interface
ADD_LIBRARY(autoffiCore src/Type.cpp)
# FIXME: boost package finder seems to be broken
TARGET_LINK_LIBRARIES(autoffiCore boost_filesystem boost_system)
ADD_LIBRARY(autoffiBin src/BinFormat.cpp)
TARGET_LINK_LIBRARIES(autoffiBin autoffiCore)
ADD_LIBRARY(autoffiClang src/ClangEngine.cpp)
llvm_map_components_to_libnames(llvm_libs all)
TARGET_LINK_LIBRARIES(autoffiClang autoffiCore ${llvm_libs} ${CLANG_LIBS})
# The C interface
ADD_LIBRARY(autoffi src/PrettyPrinter.cpp src/libautoffi.cpp)
TARGET_LINK_LIBRARIES(autoffi autoffiBin autoffiClang)
set_target_properties(autoffiCore autoffiBin autoffiClang autoffi PROPERTIES DEFINE_SYMBOL BUILDING_SHARED)
# CLI Tools
ADD_EXECUTABLE(afdump src/dump.cpp src/PrettyPrinter.cpp)
TARGET_LINK_LIBRARIES(afdump autoffiBin)
ADD_EXECUTABLE(afconvert src/convert.cpp)
TARGET_LINK_LIBRARIES(afconvert autoffiBin)
ADD_EXECUTABLE(afcompile src/tooling.cpp src/PrettyPrinter.cpp)
TARGET_LINK_LIBRARIES(afcompile autoffiClang ${CLANG_LIBS} autoffiBin)
# Testing
ENABLE_TESTING()
ADD_TEST(headerASTDump afcompile test/assets/basic.h)
test/CMakeLists.txt
CMAKE_MINIMUM_REQUIRED(VERSION 2.8)
project("AutoFFI Tests")
SET(CMAKE_CXX_STANDARD 11)
SET(GTEST_ROOT "${CMAKE_CURRENT_LIST_DIR}/gtest")
find_package(GTest REQUIRED)
INCLUDE_DIRECTORIES(${AUTOFFI_INCLUDE_DIRS} ${GTEST_INCLUDE_DIRS})
ADD_EXECUTABLE(autoffiTest libautoffi.cpp core.cpp formats.cpp clang.cpp)
TARGET_LINK_LIBRARIES(autoffiTest ${GTEST_BOTH_LIBRARIES})
SET_TARGET_PROPERTIES(autoffiTest PROPERTIES OUTPUT_NAME runtests)

Answer 1

You are keeping your main directory and your subdirectory test in completely separated projects. I suppose you are building test by invoking separately cmake and make a second time. No surprise that the include_directories of your main project as no impact on the test project.
You mention the command add_subdirectory, (that should be used to include test in from your main directory) but you are actually not using it...

Answer 2

If you're using include_directories() to add dirs, those should propagate down into targets in the test subdir. Beware though that relative paths are relative to the current source dir:
include_directories: Add include directories to the build.
include_directories([AFTER|BEFORE] [SYSTEM] dir1 dir2 ...)
Add the given directories to those the compiler uses to search for include files. Relative paths are interpreted as relative to the current source directory.
cmake docs