cmake: Download easylogging++ and use sources directly

I would like to download easylogging++ package, extract the content and then directly use easylogging++.h and easylogging++.cc in my sources.

I started with this:

ExternalProject_Add(
        easyloggingpp
        PREFIX ${CMAKE_CURRENT_SOURCE_DIR}/downloads
        URL https://github.com/muflihun/easyloggingpp/archive/v9.96.4.tar.gz
        INSTALL_COMMAND mkdir -p ${CMAKE_CURRENT_BINARY_DIR}/external/easyloggingpp && cp src/easyloggingpp-9.96.4/src/* ${CMAKE_CURRENT_BINARY_DIR}/external/easyloggingpp/)
include_directories(${CMAKE_CURRENT_BINARY_DIR}/external/easyloggingpp) 
set(easylogging ${CMAKE_CURRENT_BINARY_DIR}/external/easyloggingpp/easylogging++.cc)

..

add_dependencies(myproject easyloggingpp)

This creates downloads/ directory in my project, but it's empty and no files appear in external/ directory, even the directory itself is not created.

How can I achieve downloading this package and directly merging its sources with mine? I would like to achieve something similar to bazel's new_http_archive.