CMake: Only link against libpng when needed

When using the system `SDL(2)_image`, we do not need to link against
libpng.

3rdParty/SDL_image/CMakeLists.txt

@@ -1,5 +1,20 @@
-include(FetchContent_MakeAvailableExcludeFromAll)
+option(DEVILUTIONX_SYSTEM_LIBPNG "Use system-provided libpng" ON)
+cmake_dependent_option(DEVILUTIONX_STATIC_LIBPNG "Link static libpng" OFF
+                      "DEVILUTIONX_SYSTEM_LIBPNG AND NOT DIST" ON)
 
+if(DEVILUTIONX_SYSTEM_LIBPNG)
+  find_package(PNG QUIET)
+  if(PNG_FOUND)
+    message("-- Found png ${PNG_VERSION_STRING}")
+  else()
+    message("-- Suitable system png package not found, will use png from source")
+  endif()
+endif()
+if(NOT PNG_FOUND)
+  add_subdirectory(../libpng libpng)
+endif()
+
+include(FetchContent_MakeAvailableExcludeFromAll)
 include(FetchContent)
 if(USE_SDL1)
   FetchContent_Declare(SDL_image

CMake/ctr/modules/FindPNG.cmake

@@ -63,3 +63,4 @@ set(PNG_PROCESS_LIBS PNG_LIBRARY LIBM_LIBRARY)
 libfind_process(PNG)
 
 try_add_imported_target(PNG m 3ds::zlib)
+add_library(PNG::PNG ALIAS 3ds::png)

CMakeLists.txt

@@ -89,10 +89,6 @@ option(DEVILUTIONX_SYSTEM_LIBFMT "Use system-provided libfmt" ON)
 cmake_dependent_option(DEVILUTIONX_STATIC_LIBFMT "Link static libfmt" OFF
                        "DEVILUTIONX_SYSTEM_LIBFMT AND NOT DIST" ON)
 
-option(DEVILUTIONX_SYSTEM_LIBPNG "Use system-provided libpng" ON)
-cmake_dependent_option(DEVILUTIONX_STATIC_LIBPNG "Link static libpng" OFF
-                       "DEVILUTIONX_SYSTEM_LIBPNG AND NOT DIST" ON)
-
 option(DEVILUTIONX_SYSTEM_SDL_IMAGE "Use system-provided SDL_image" ON)
 cmake_dependent_option(DEVILUTIONX_STATIC_SDL_IMAGE "Link static SDL_image" OFF
                        "DEVILUTIONX_SYSTEM_SDL_IMAGE AND NOT DIST" ON)
@@ -260,21 +256,6 @@ if(NOT fmt_FOUND)
   add_subdirectory(3rdParty/libfmt)
 endif()
 
-if(DEVILUTIONX_SYSTEM_LIBPNG)
-  find_package(PNG QUIET)
-  if(PNG_FOUND)
-    message("-- Found png ${PNG_VERSION_STRING}")
-  else()
-    message("-- Suitable system png package not found, will use png from source")
-  endif()
-  if(NOT TARGET PNG::PNG AND TARGET 3ds::png)
-    add_library(PNG::PNG ALIAS 3ds::png)
-  endif()
-endif()
-if(NOT PNG_FOUND)
-  add_subdirectory(3rdParty/libpng)
-endif()
-
 if(ANDROID)
   add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/android-project/3rdParty/SDL2)
 elseif(USE_SDL1)
@@ -899,7 +880,6 @@ if(NOT NONET)
 endif()
 
 target_link_libraries(libdevilutionx PUBLIC fmt::fmt)
-target_link_libraries(libdevilutionx PUBLIC PNG::PNG)
 
 genex_for_option(DEBUG)
 target_compile_definitions(libdevilutionx PUBLIC "$<${DEBUG_GENEX}:_DEBUG>")