Cmake: build using FetchContent

[mariuszmaximus]

I try build libpng using Cmake::FetchContent

cmake_minimum_required(VERSION 3.19)
project(png-usage)

# more logging from FetchContent
set(FETCHCONTENT_QUIET OFF)

include(FetchContent)

FetchContent_Declare(
  zlib_repo
  GIT_REPOSITORY "https://github.com/madler/zlib"
)
FetchContent_MakeAvailable(zlib_repo)

# app_zlib
add_executable(app_zlib app_z.cpp)
target_link_libraries(app_zlib zlib)

FetchContent_Declare(
  libpng
  GIT_REPOSITORY "https://github.com/glennrp/libpng.git"
)
set(PNG_TESTS OFF)
FetchContent_MakeAvailable(libpng)


# app_png.
add_executable(app_png app_png.cpp)
target_link_libraries(app_png png_static)

But i have error: Could NOT find ZLIB (missing: ZLIB_LIBRARY ZLIB_INCLUDE_DIR)

Is there any way to fix this?
Where is the problem? In libpng or zlib?

I tested Linux (gcc), Windows (msvc, msys2-clang)
It doesn't work anywhere

full repo https://github.com/mariuszmaximus/libpng_usage.git

[Epixu]

ZLIB and PNG have horrid modern CMake support. It is generally done by setting ZLIB_ROOT nowadays, however it doesn't play along with FetchContent at all. You have to relay your fetched ZLIB towards your fetched PNG.

Here's how I managed to do it

^ Works by creating a local ALIAS target called zlibstatic, and then works around find_pakage(ZLIB) in png's cmake config by setting ZLIB_LIBRARY and ZLIB_INCLUDE_DIR manually. ZLIB_ROOT is still required for some reason. I've tried many other things, but this seems to be the only thing that works. If anyone gets a better idea, please share. I've tested building on windows/linux, with g++/clang/cl/clang-cl.

PS: fetch_external_module is just a wrapper for FetchContent_Declare and FetchContent_MakeAvailable - you can find it here.