How to add source files to an external project in CMake?

I want to integrate SQLite into my project using ExternalProject_Add.

cmake_minimum_required(VERSION 2.8.8)
include(ExternalProject)

# Download, configure, build and install SQLite
ExternalProject_Add(SQLite
    PREFIX            ${CMAKE_SOURCE_DIR}
    TMP_DIR           ${CMAKE_SOURCE_DIR}/temp
    STAMP_DIR         ${CMAKE_SOURCE_DIR}/stamp
    #--Download step--------------
    DOWNLOAD_DIR      ${CMAKE_SOURCE_DIR}/download
    URL               http://www.sqlite.org/2014/sqlite-autoconf-3080704.tar.gz
    URL_HASH          SHA1=70ca0b8884a6b145b7f777724670566e2b4f3cde
    #--Update/Patch step----------
    UPDATE_COMMAND    ""
    #--Configure step-------------
    SOURCE_DIR        ${CMAKE_SOURCE_DIR}/source
    CONFIGURE_COMMAND "" # How to add sqlite3.c to the target here?
    #--Build step-----------------
    BINARY_DIR        ${CMAKE_SOURCE_DIR}/build
    BUILD_COMMAND     "cmake --build ."
    #--Install step---------------
    INSTALL_DIR       ${CMAKE_SOURCE_DIR}/install
)

The build command would use the native compiler to build all source files added to the target SQLite. However, there are non. How can I add the only source file sqlite3.c to the external project within the CONFIGURE_COMMAND?

Solution

ExternalProject_Add assumes that the project you want to pull in already ships with a (possibly complex, possibly non-CMake-based) working build system.

You have two possibilities here:

You can stick with the amalgamated autoconf version of sqlite that you are currently using. In that case the CONFIGURE_COMMAND would invoke configure and the BUILD_COMMAND would invoke make. Note that this approach will not be portable to platforms that do not have autoconf installed.
You can switch to the bare-source amalgamated version of sqlite and provide your own CMakeLists.txt for building. Since sqlite can be built with a minimum of configuration and the amalgamation only consists of a single source and header file, this is not as hard as it may sound. In this case you can simply invoke cmake for configuation and building.

Note however that you cannot provide this information in-line with ExternalProject_Add. You will need an external build script, whether that is CMake, autoconf or something else.