CMakeLists.txt not in root directory, can't do an automatic build - cmake

Question

I have an issue with a repository not having its CMakeLists in the root directory, namely https://github.com/lz4/lz4
The CMakeLists.txt is in the subfolder contrib/cmake_unofficial.
I already checked similar questions on SO (Is it possible to have cmake build file (CMakeLists.txt) not in root in CLion, cmake - CMakeLists.txt is not in root folder (but is included in source)), but they only provide alternatives, and not a solution applicable to my situation.
Heres the cmake module I came up with:
if(ENABLE_LZ4)
message(STATUS "Using LZ4.")
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DENABLE_LZ4")
# Enable ExternalProject CMake module
include(ExternalProject)
set(LZ4_SOURCE_DIR ${CMAKE_CURRENT_BINARY_DIR}/lz4)
set(LZ4_BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR}/lib)
# Download and install lz4
ExternalProject_Add(
lz4
GIT_REPOSITORY https://github.com/lz4/lz4.git
GIT_TAG dev
SOURCE_DIR ${LZ4_SOURCE_DIR}
BINARY_DIR ${LZ4_BINARY_DIR}
INSTALL_COMMAND ""
CMAKE_ARGS
${LZ4_SOURCE_DIR}/contrib/cmake_unofficial
-DCMAKE_C_COMPILER=${CMAKE_C_COMPILER}
)
# Get lz4 source and binary directories from CMake project
ExternalProject_Get_Property(lz4 source_dir binary_dir)
# Create a liblz4 target to be used as a dependency by the program
add_library(liblz4 IMPORTED SHARED GLOBAL)
add_dependencies(liblz4 lz4)
include_directories(
${LZ4_SOURCE_DIR}/lib
)
set(LZ4_LIB ${LZ4_BINARY_DIR}/liblz4.so)
else()
message(STATUS "Not using LZ4.")
set(LZ4_LIB "")
endif()
Here the complete error output:
[ 0%] Performing update step for 'lz4'
Current branch dev is up to date.
[ 1%] Performing configure step for 'lz4'
CMake Error: The source directory "/****/build/lz4" does not appear to contain CMakeLists.txt.
Specify --help for usage, or press the help button on the CMake GUI.
CMakeFiles/lz4.dir/build.make:105: recipe for target 'lz4-prefix/src/lz4-stamp/lz4-configure' failed
make[2]: *** [lz4-prefix/src/lz4-stamp/lz4-configure] Error 1
CMakeFiles/Makefile2:72: recipe for target 'CMakeFiles/lz4.dir/all' failed
make[1]: *** [CMakeFiles/lz4.dir/all] Error 2
Makefile:94: recipe for target 'all' failed
make: *** [all] Error 2
I tried adding the path 'contrib/cmake_unofficial' to the CMAKE_ARGS variable (as seen in the module above), but it does not work (seems to be ignored?).
I also tried using PATCH_COMMAND to copy the CMakeLists.txt to the root before the build starts, but the relative paths of the file get messed up.
In other words, i need the cmake command to be called to build the library to be : cmake contrib/cmake_unofficial.
I also tried using CONFIGURE_COMMAND for this, but keep getting a file not found error for some reason (even though the path is correct).
The module has some other issues too, but I'm only interested in the non-root CMakeLists.
Thanks in advance!

Answer 1

ExternalProject separates download and source directories:
DOWNLOAD_DIR - a directory where downloading step is performed
SOURCE_DIR - a directory used as a source one when configuration step is performed
When use git for extract the project, note that git clone is called from the download directory, where it creates new directory with a project sources.
set(LZ4_DOWNLOAD_DIR ${CMAKE_CURRENT_BINARY_DIR}/lz4)
# Set a source dir based on the download one.
set(LZ4_SOURCE_DIR ${LZ4_DOWNLOAD_DIR}/lz4/contrib/cmake_unofficial)
set(LZ4_BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR}/lib)
# Download and install lz4
ExternalProject_Add(
lz4
DOWNLOAD_DIR ${LZ4_DOWNLOAD_DIR} # Set download directory explicitely
GIT_REPOSITORY https://github.com/lz4/lz4.git
GIT_TAG dev
SOURCE_DIR ${LZ4_SOURCE_DIR}
BINARY_DIR ${LZ4_BINARY_DIR}
...
)

Answer 2

Probably not the correct way to do it, but this seems to work for me.
I used the CONFIGURE_COMMAND to call cmake on the correct directory.
Then use BUILD_COMMAND to call make
So essentially, it breaks down to this:
ExternalProject_Add(
lz4
GIT_REPOSITORY https://github.com/lz4/lz4.git
GIT_TAG dev
SOURCE_DIR ${LZ4_SOURCE_DIR}
BINARY_DIR ${LZ4_BINARY_DIR}
CONFIGURE_COMMAND cmake ${LZ4_SOURCE_DIR}/contrib/cmake_unofficial
BUILD_COMMAND make
INSTALL_COMMAND ""
CMAKE_ARGS
-DCMAKE_C_COMPILER=${CMAKE_C_COMPILER}
)