CMake ExternalProject_Add use automatically set variables

According to documentation, ExternalProject_Add sets unset directory variables itself.

If any of the above ..._DIR options are not specified, their defaults are computed as follows. If the PREFIX option is given or the EP_PREFIX directory property is set, then an external project is built and installed under the specified prefix:
TMP_DIR      = <prefix>/tmp
STAMP_DIR    = <prefix>/src/<name>-stamp
DOWNLOAD_DIR = <prefix>/src
SOURCE_DIR   = <prefix>/src/<name>
BINARY_DIR   = <prefix>/src/<name>-build
INSTALL_DIR  = <prefix>

In the following example I want to add COIN-OR-CLP as an external project with a custom CONFIGURE_COMMAND because CLP uses Autotools. This works.

include(ExternalProject)
ExternalProject_Add(${EXT_PROJ}
                    PREFIX ${EXT_PROJ}
                    SVN_REPOSITORY https://projects.coin-or.org/svn/Clp/stable/1.16/
                    UPDATE_COMMAND svn cleanup  # Beause otherwise svn will fail on consecutive runs
                    CONFIGURE_COMMAND ${CMAKE_CURRENT_BINARY_DIR}/${EXT_PROJ}/${EXT_PROJ}/src/${EXT_PROJ}/configure

)

However I think it'd be cleaner to use the generated paths instead manually passing it.

Is there a way to use ${SOUCRE_DIR} in ExternalProject_Add without setting it explicitly?

Solution

All directories you refer to can be used in ExternalProject_Add COMMAND's with <...> notation:

CONFIGURE_COMMAND <SOURCE_DIR>/configure

For some reason, this feature is described at the end of ExternalProject_Add_Step function's description, not in the function ExternalProject_Add.