Fix macOS build with the SDL2 2.24.0

SDL2 2.24.0 has a bug in its cmake for macOS: https://github.com/libsdl-org/SDL/issues/6119

This bug has already been fixed upstream. This is a workaround until the
next version of SDL2 is released.

CMake/Dependencies.cmake

@@ -31,7 +31,9 @@ else()
   if(DEVILUTIONX_SYSTEM_SDL2)
     find_package(SDL2 REQUIRED)
     if(TARGET SDL2::SDL2)
-      set(SDL2_MAIN SDL2::SDL2main)
+      if(TARGET SDL2::SDL2main)
+        set(SDL2_MAIN SDL2::SDL2main)
+      endif()
     elseif(TARGET SDL2::SDL2-static)
       # On some distros, such as vitasdk, only the SDL2::SDL2-static target is available.
       # Alias to SDL2::SDL2 because some finder scripts may refer to SDL2::SDL2.
@@ -39,7 +41,9 @@ else()
         # Aliasing local targets is not supported on CMake < 3.18, so make it global.
         set_target_properties(SDL2::SDL2-static PROPERTIES IMPORTED_GLOBAL TRUE)
       endif()
-      set(SDL2_MAIN SDL2::SDL2main)
+      if(TARGET SDL2::SDL2main)
+        set(SDL2_MAIN SDL2::SDL2main)
+      endif()
     else()
       # Assume an older Debian derivate that comes with an sdl2-config.cmake
       # that only defines `SDL2_LIBRARIES` (as -lSDL2) and `SDL2_INCLUDE_DIRS`.
@@ -54,7 +58,9 @@ else()
     find_package(SDL2 REQUIRED)
   else()
     add_subdirectory(3rdParty/SDL2)
-    set(SDL2_MAIN SDL2::SDL2main)
+    if(TARGET SDL2::SDL2main)
+      set(SDL2_MAIN SDL2::SDL2main)
+    endif()
     list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/3rdParty/SDL2/CMake")
   endif()
 endif()