Avoid checking for HAVE_POSIX_SPAWN on Android

This makes the CMake build closer to SDL_build_config_android.h
as both will now use SDL_PROCESS_DUMMY.

(cherry picked from commit cd21bbe7968419cc6bcaeca8b95081b1a39edeb3)

CMakeLists.txt

@@ -2962,7 +2962,7 @@ if(WINDOWS)
   sdl_glob_sources("${SDL3_SOURCE_DIR}/src/process/windows/*.c")
   set(SDL_PROCESS_WINDOWS 1)
   set(HAVE_SDL_PROCESS TRUE)
-else()
+elseif(NOT ANDROID)
   check_c_source_compiles("
 #include <spawn.h>
 #include <unistd.h>