How to download a toolchain for cross compilation in cmake from separate file?

Question

I have a project with a CMakeLists.txt files in the root and the project compiles fine on Linux and OSX. Now I want to cross compile it for MIPS OpenWRT. I would like to automate it as much as possible, so I would use following code to download the toolchain and set the compiler variables:

ExternalProject_Add(ar71xx-toolchain
    PREFIX "${PROJECT_BINARY_DIR}/external/openwrt"
    URL "http://downloads.openwrt.org/barrier_breaker/14.07/ar71xx/generic/OpenWrt-Toolchain-ar71xx-for-mips_34kc-gcc-4.8-linaro_uClibc-0.9.33.2.tar.bz2"
    UPDATE_COMMAND ""
    PATCH_COMMAND ""
    BUILD_COMMAND ""
    CONFIGURE_COMMAND ""
    INSTALL_COMMAND ""
)
ExternalProject_Get_Property(ar71xx-toolchain SOURCE_DIR)
SET(CMAKE_C_COMPILER ${SOURCE_DIR}/toolchain-mips_34kc_gcc-4.8-linaro_uClibc-0.9.33.2/bin/mips-openwrt-linux-gcc)
SET(CMAKE_CXX_COMPILER ${SOURCE_DIR}/toolchain-mips_34kc_gcc-4.8-linaro_uClibc-0.9.33.2/bin/mips-openwrt-linux-g++)
SET(CMAKE_STRIP ${SOURCE_DIR}/toolchain-mips_34kc_gcc-4.8-linaro_uClibc-0.9.33.2/bin/mips-openwrt-linux-strip)

I thought that I can put it in a separate toolchain file and pass it with -DCMAKE_TOOLCHAIN_FILE, but it seems that ExternalProject_Add is not executed inside the toolchain file. I would like to avoid putting the toolchain download step into the main CMakeLists.txt since it's actually not essential for the project itself and would require doing the same for each target platform... So is there a way to define optional steps for a current cross compile build and pass it somehow as command line parameter to be executed before the main project build?

UPDATE: Based on Tsyvarev's answer that works for me in the toolchain file:

set(CMAKE_SYSTEM_NAME Linux)
set(TOOLCHAIN_DIR ${PROJECT_BINARY_DIR}/external/openwrt/toolchain)

if(NOT EXISTS ${TOOLCHAIN_DIR})
    file(DOWNLOAD http://downloads.openwrt.org/barrier_breaker/14.07/ar71xx/generic/OpenWrt-Toolchain-ar71xx-for-mips_34kc-gcc-4.8-linaro_uClibc-0.9.33.2.tar.bz2 ${TOOLCHAIN_DIR}/toolchain.tar.bz2 SHOW_PROGRESS)
    execute_process(COMMAND tar --strip-components=2 -xjf ${TOOLCHAIN_DIR}/toolchain.tar.bz2 WORKING_DIRECTORY ${TOOLCHAIN_DIR})
    execute_process(COMMAND rm ${TOOLCHAIN_DIR}/toolchain.tar.bz2)
endif()

SET(CMAKE_C_COMPILER ${TOOLCHAIN_DIR}/bin/mips-openwrt-linux-gcc)
SET(CMAKE_CXX_COMPILER ${TOOLCHAIN_DIR}/bin/mips-openwrt-linux-g++)
SET(CMAKE_STRIP ${TOOLCHAIN_DIR}/bin/mips-openwrt-linux-strip)
SET(CMAKE_FIND_ROOT_PATH  ${TOOLCHAIN_DIR})
SET(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
SET(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
SET(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)

There is one issue when passing -DCMAKE_TOOLCHAIN_FILE as CMAKE parameter to other projects added with ExternalProject_Add. Because of it's own ${PROJECT_BINARY_DIR} it will download the toolchain again. But this is another problem...

Answer 1

ExternalProject_add executes all steps at build time, not at configuration time.

For download file you can use file(DOWNLOAD ...) command. For extract files from archive just use execute_process with appropriate command.