CMake External Project not Downloading - cmake

Question

I am using CMake 3.5.1 with CLion, and I am attempting to download and install an external project from a URL. My CMakeLists.txt include:
include(ExternalProject)
set(EXTERNAL ${PROJECT_SOURCE_DIR}/external)
ExternalProject_Add(eigen_test
PREFIX ${EXTERNAL}/eigen
DOWNLOAD_DIR ${EXTERNAL}/eigen/download
SOURCE_DIR ${EXTERNAL}/eigen/src
BINARY_DIR ${EXTERNAL}/eigen/build
INSTALL_DIR ${EXTERNAL}/eigen/install
URL http://bitbucket.org/eigen/eigen/get/3.2.4.tar.gz
URL_MD5 4d0d77e06fef87b4fcd2c9b72cc8dc55
CONFIGURE_COMMAND cd <BINARY_DIR> && cmake -D CMAKE_INSTALL_PREFIX=$<INSTALL_DIR> <SOURCE_DIR>
)
When building, this directory structure is is created inside ${PROJECT_SOURCE_DIR}/external/
Although the cmake files exist to download and extract the project, these actions never occur. What am I doing wrong here?

Answer 1

I don't see a reason to pollute your project by downloading and building the library in ${PROJECT_SOURCE_DIR}, I always do that in ${CMAKE_BINARY_DIR}.
The eigen library uses CMakeLists.txt, it will work seamlessly as external project, you don't have to extensively configure each step. So CONFIGURE_COMMAND can be removed and the install prefix set with CMAKE_ARGS -DCMAKE_INSTALL_PREFIX=...
This is the simplified and working configuration:
include(ExternalProject)
set(EXTERNALS_DIR ${CMAKE_BINARY_DIR}/external)
ExternalProject_Add(eigen_test
PREFIX ${EXTERNALS_DIR}
URL http://bitbucket.org/eigen/eigen/get/3.2.4.tar.gz
URL_MD5 4d0d77e06fef87b4fcd2c9b72cc8dc55
CMAKE_ARGS -DCMAKE_INSTALL_PREFIX=${EXTERNALS_DIR}/installed
)
How to use the library:
Add ${EXTERNALS_DIR}/installed/include to include directories
Build the eigen_test target