CMakeLists.txt

project (external_project)
cmake_minimum_required (VERSION 3.6)

set (SOURCE_DIR ${CMAKE_SOURCE_DIR}/src)
set (EXECUTABLE_OUTPUT_PATH ${CMAKE_SOURCE_DIR}/output)

list (APPEND HELLO_SOURCES ${SOURCE_DIR}/hello.c)

include (ExternalProject)

set (JASSON_INSTALL_DIR ${CMAKE_CURRENT_BINARY_DIR})
set (JASSON_CMAKE_ARGS -DCMAKE_INSTALL_PREFIX=${JASSON_INSTALL_DIR} -DJASSON_BUILD_DOCS=OFF)

ExternalProject_Add (jasson
  GIT_REPOSITORY https://github.com/akheron/jansson.git
  INSTALL_DIR ${JASSON_INSTALL_DIR}
  PREFIX ${JASSON_INSTALL_DIR}
  CMAKE_ARGS ${JASSON_CMAKE_ARGS}
)
ExternalProject_Get_Property (jasson INSTALL_DIR)

include_directories (${INSTALL_DIR}/include)

add_executable (hello ${HELLO_SOURCES})

add_dependencies (hello jasson)