Passing variables and their values from one ExternalProject dependency to another using cmake (in the context of zlib and libpng) - cmake

Question

I am still at the beginning of my journey regarding ExternalProject. I have two projects used by my main project as dependencies: libpng and zlib. The former depends on the latter.
Inside the official CMakeLists.txt of libpng I see that there is the option PNG_BUILD_ZLIB, which you have to disable if you want to use a custom version of zlib. Currently my two externals look like this:
ExternalProject_Add(zlib
PREFIX ${CMAKE_CURRENT_BINARY_DIR}/deps/zlib
INSTALL_DIR ${CMAKE_INSTALL_PREFIX}
DOWNLOAD_DIR ""
TMP_DIR ${DEPS_TMP}/zlib
STAMP_DIR ${DEPS_STAMP}/zlib
LOG_DIR ${DEPS_LOG}/zlib
BINARY_DIR ${DEPS_BUILD}/zlib
SOURCE_DIR ${DEPS}/zlib
INSTALL_COMMAND "${CMAKE_COMMAND}" -E echo "Skipping install step for dependency zlib"
INSTALL_PREFIX ${CMAKE_INSTALL_PREFIX}
BUILD_ALWAYS OFF
)
# Set zlib variables required by libpng
ExternalProject_Get_property(zlib BINARY_DIR)
ExternalProject_Get_property(zlib SOURCE_DIR)
set(zlib_DIR "${BINARY_DIR}/Debug" CACHE PATH "zlib dir" FORCE)
set(ZLIB_ROOT ${SOURCE_DIR} CACHE PATH "zlib root" FORCE) # ${zlib_DIR}
set(ZLIB_INCLUDE_DIRS ${SOURCE_DIR} CACHE PATH "zlib include dir" FORCE)
if(WIN32)
set(ZLIB_LIBRARY ${zlib_DIR}/zlibd.dll CACHE FILEPATH "zlib dynamic library" FORCE)
set(ZLIB_LIBRARIES ${zlib_DIR}/zlibd.dll CACHE FILEPATH "zlib dynamic library" FORCE)
else()
set(ZLIB_LIBRARY ${zlib_DIR}/libzlibd.so CACHE FILEPATH "zlib dynamic library" FORCE)
set(ZLIB_LIBRARIES ${zlib_DIR}/libzlibd.so CACHE FILEPATH "zlib dynamic library" FORCE)
endif()
ExternalProject_Add(libpng
PREFIX ${CMAKE_CURRENT_BINARY_DIR}/deps/png
DEPENDS zlib
INSTALL_DIR ${CMAKE_INSTALL_PREFIX}
DOWNLOAD_DIR ""
TMP_DIR ${DEPS_TMP}/png
STAMP_DIR ${DEPS_STAMP}/png
LOG_DIR ${DEPS_LOG}/png
BINARY_DIR ${DEPS_BUILD}/png
SOURCE_DIR ${DEPS}/libpng
INSTALL_COMMAND "${CMAKE_COMMAND}" -E echo "Skipping install step for dependency libpng"
INSTALL_PREFIX ${CMAKE_INSTALL_PREFIX}
BUILD_ALWAYS OFF
CMAKE_ARGS
"-DPNG_BUILD_ZLIB=1" # Use zlib built as a dependency of this project
"-DPNG_SHARED=1"
"-DPNG_STATIC=0"
"-DPNG_TESTS=0"
"-DPNG_EXECUTABLES=0"
"-DPNG_HARDWARE_OPTIMIZATIONS=1"
"-DPNG_DEBUG=0"
)
where
${DEPS} points at a directory where currently every dependency's source code resides and is manually downloaded using git submodules commands
${DEPS_xyz} points at a ExternalProject directory for a specific purpose (logs, stamps, build files, temporary files etc.) and all are placed inside the build directory of the whole project.
The problem I am having is that I am failing at passing the include directory to libpng and this results in the header zlib.h not being found during the build step for that dependency. Apparently
set(ZLIB_INCLUDE_DIRS ${SOURCE_DIR} CACHE PATH "zlib include dir" FORCE)
isn't really passed to the dependency (but in itself it is pointing at the correct path on the filesystem where the header file is included).
Do I have to add the include directory as a CMAKE_ARGS entry or is there some other easier way?
I also checked here but the suggested solution (I just passed the stuff directly into my CMAKE_ARGS)
CMAKE_ARGS
"-DPNG_BUILD_ZLIB=1" # Use zlib built as a dependency of this project
"-DZLIB_INCLUDE_DIRS=\"${${ZLIB_INCLUDE_DIRS}}\"" # Pass the ZLIB_INCLUDE_DIRS to the dependency using the value of the variable set above
"-DPNG_SHARED=1"
"-DPNG_STATIC=0"
"-DPNG_TESTS=0"
"-DPNG_EXECUTABLES=0"
"-DPNG_HARDWARE_OPTIMIZATIONS=1"
"-DPNG_DEBUG=0"
doesn't work.