if(NOT NACL_FOUND)

    message("libnacl not found, will be downloaded and compiled.")

    include(ExternalProject)

    # Without this, the build doesn't happen until link time.

    include_directories(${NACL_USE_FILES})

    # the name of the tag

    set(tag "cnacl-e551eb9a55897c651a6688223b8734223f990175.tar.gz")

    # Configure cnacl

    set(cNaClConfig "

        add_definitions(\"-fPIC\")

        set(MY_CMAKE_ASM_FLAGS \"-fPIC\")

        set(CMAKE_ASM_COMPILER \"${CMAKE_C_COMPILER}\")

    ")

    file(WRITE ${CMAKE_BINARY_DIR}/cNaClConfig.cmake "${cNaClConfig}")

    set(cmakeArgs "-DCNACL_CONFIG_SCRIPT=${CMAKE_BINARY_DIR}/cNaClConfig.cmake")

    if (CMAKE_TOOLCHAIN_FILE)

        get_filename_component(toolchainFilePath "${CMAKE_TOOLCHAIN_FILE}" REALPATH)

        list(APPEND cmakeArgs "-DCMAKE_TOOLCHAIN_FILE=${toolchainFilePath}")

    endif()

    set(file ${CMAKE_BINARY_DIR}/nacl_ep-prefix/src/${tag})

    set(AssertSHA256 ${CMAKE_SOURCE_DIR}/cmake/modules/AssertSHA256.cmake)

    set(check ${CMAKE_COMMAND} -DFILE=${file} -DEXPECTED=${hash} -P ${AssertSHA256})

    set(url "${CMAKE_SOURCE_DIR}/cmake/externals/${tag}")

    if(NOT EXISTS "${url}")

        set(url ${CMAKE_BINARY_DIR}/nacl_ep-prefix/src/${tag})

    endif()

    file(MAKE_DIRECTORY "${CMAKE_BINARY_DIR}/nacl_build")

    ExternalProject_Add(nacl_ep

        URL ${url}

        SOURCE_DIR "${CMAKE_BINARY_DIR}/nacl"

        BINARY_DIR "${CMAKE_BINARY_DIR}/nacl_build"

        CMAKE_ARGS ${cmakeArgs}

        INSTALL_COMMAND ""

        UPDATE_COMMAND ""

        PATCH_COMMAND ""

    )

    add_custom_command(

        OUTPUT ${CMAKE_BINARY_DIR}/nacl/build/nacl_test.out

        COMMAND ${CMAKE_COMMAND} -P ${CMAKE_SOURCE_DIR}/cmake/modules/TestNACL.cmake

    )

    add_custom_target(nacl_test DEPENDS ${CMAKE_BINARY_DIR}/nacl/build/nacl_test.out)

    add_dependencies(nacl_test nacl_ep)

    # The source of this uglyness is a limit on adding dependencies to imported libraries.

    # see: http://www.cmake.org/Bug/print_bug_page.php?bug_id=10395

    # It's fixed in cmake 2.8.4 but it would be nice to continue supporting 2.8.2 and 2.8.3

    if(NOT EXISTS ${CMAKE_BINARY_DIR}/DoNothing.c)

        file(WRITE ${CMAKE_BINARY_DIR}/DoNothing.c "int DoNothing() { return 0; }\n")

    endif()

    add_library(nacl_test_dependency ${CMAKE_BINARY_DIR}/DoNothing.c)

    add_dependencies(nacl_test_dependency nacl_test)

    add_library(nacl STATIC IMPORTED)

    set_property(TARGET nacl

        PROPERTY IMPORTED_LOCATION ${CMAKE_BINARY_DIR}/nacl_build/libnacl.a)

    set(NACL_INCLUDE_DIRS "${CMAKE_BINARY_DIR}/nacl_build/include/")

    set(NACL_LIBRARIES nacl nacl_test_dependency)

    set(NACL_FOUND TRUE)

endif()