vscode intellisense cmake-tools with external library - visual-studio-code

Question

I'm trying to configure a template project for my STM32 blue pill board with CMake and cmake-tools for Visual Studio Code. It builds just fine, but the Intellisense feature of cmake-tools seems not to work with an external library (libopencm3 in this case).
This is most likely a configuration error on my side. Following my CMakeLists.txt. The toolchain is defined in a seperate .cmake file.
set(PRJ_NAME STM32_Template_Project)
cmake_minimum_required(VERSION 3.6)
project(${PRJ_NAME} C ASM)
set(COMMON_FLAGS "-mcpu=${MCU_ARCH} -mthumb -mthumb-interwork -mfloat-abi=${MCU_FLOAT_ABI} -ffunction-sections -fdata-sections -g -fno-common -fmessage-length=0 -DSTM32F1")
if (MCU_FLOAT_ABI STREQUAL hard)
set(COMMON_FLAGS "${COMMON_FLAGS} -mfpu=${MCU_FPU}")
endif ()
set(CMAKE_CXX_FLAGS "${COMMON_FLAGS} -std=c++11")
set(CMAKE_C_FLAGS "${COMMON_FLAGS} -std=gnu99")
set(CMAKE_EXE_LINKER_FLAGS "-Wl,-gc-sections -T ${CMAKE_SOURCE_DIR}/${MCU_LINKER_SCRIPT}")
set(DEFINITIONS "${DEFINITIONS} -D${MCU_LINE}")
set(LIBOPENCM3_DIR "../libopencm3")
#headers
include_directories(sys)
include_directories(Inc)
include_directories(${LIBOPENCM3_DIR}/include)
# libopencm3
# Make sure that git submodule is initialized and updated
if (NOT EXISTS ${LIBOPENCM3_DIR}/Makefile)
message(FATAL_ERROR "libopencm3 submodule not found. Initialize with 'git submodule update --init' in the source directory")
endif()
add_custom_target(libopencm3 make TARGETS=stm32/f1 WORKING_DIRECTORY ${LIBOPENCM3_DIR})
link_directories(${LIBOPENCM3_DIR}/lib)
#files
file(GLOB_RECURSE USER_SOURCES Src/*.c)
set(CMSIS_STARTUP startup_${MCU_LINE}.s)
set(SOURCE_FILES ${USER_SOURCES} ${CMSIS_STARTUP} ${MCU_LINKER_SCRIPT})
# target
add_executable(${PROJECT_NAME}.elf ${SOURCE_FILES})
target_link_libraries(${PROJECT_NAME}.elf opencm3_stm32f1)
set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -Wl,-Map=${PROJECT_SOURCE_DIR}/build/${PROJECT_NAME}.map")
set(HEX_FILE ${PROJECT_SOURCE_DIR}/build/${PROJECT_NAME}.hex)
set(BIN_FILE ${PROJECT_SOURCE_DIR}/build/${PROJECT_NAME}.bin)
add_custom_command(TARGET ${PROJECT_NAME}.elf POST_BUILD
COMMAND ${CMAKE_OBJCOPY} -Oihex $<TARGET_FILE:${PROJECT_NAME}.elf> ${HEX_FILE}
COMMAND ${CMAKE_OBJCOPY} -Obinary $<TARGET_FILE:${PROJECT_NAME}.elf> ${BIN_FILE}
COMMENT "Building ${HEX_FILE} \nBuilding ${BIN_FILE}")
I am not very experienced with CMake and cmake-tools.