cmake: use CMAKE_ANDROID_NDK variable as root of Android ndk

This variable is always defined when targeting Android:
- when using the toolchain file shipped along the ndk
  (using  -DCMAKE_TOOLCHAIN_FILE=$ENV{ANDROID_NDK_HOME}/build/cmake/android.toolchain.cmake)
- when using the toolchain file provided by CMake
  (using -DCMAKE_SYSTEM_NAME=Android)

CMakeLists.txt

@@ -1294,8 +1294,8 @@ if(ANDROID)
   list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_LIST_DIR}/cmake/android")
 
   sdl_glob_sources("${SDL3_SOURCE_DIR}/src/core/android/*.c")
-  sdl_sources("${ANDROID_NDK}/sources/android/cpufeatures/cpu-features.c")
-  set_property(SOURCE "${ANDROID_NDK}/sources/android/cpufeatures/cpu-features.c" APPEND_STRING PROPERTY COMPILE_FLAGS " -Wno-declaration-after-statement")
+  sdl_sources("${CMAKE_ANDROID_NDK}/sources/android/cpufeatures/cpu-features.c")
+  set_property(SOURCE "${CMAKE_ANDROID_NDK}/sources/android/cpufeatures/cpu-features.c" APPEND_STRING PROPERTY COMPILE_FLAGS " -Wno-declaration-after-statement")
 
   sdl_glob_sources("${SDL3_SOURCE_DIR}/src/misc/android/*.c")
   set(HAVE_SDL_MISC TRUE)
@@ -3294,7 +3294,7 @@ else()
 endif()
 
 if(ANDROID)
-  sdl_include_directories(PRIVATE SYSTEM "${ANDROID_NDK}/sources/android/cpufeatures")
+  sdl_include_directories(PRIVATE SYSTEM "${CMAKE_ANDROID_NDK}/sources/android/cpufeatures")
 endif()
 
 if(APPLE)