Add upgrading note about Autoconf cache variables (php#14985)

[skip ci]

Author: petk

UPGRADING.INTERNALS

@@ -180,6 +180,18 @@ PHP 8.4 INTERNALS UPGRADE NOTES
      extension.
    - Removed BC enable_pear variable check due to --enable-pear configure option
      once used (use with_pear variable name).
+   - Cache variables synced to php_cv_* naming scheme. If you use them for
+     advanced cross-compilation, these were renamed:
+       - ac_cv_copy_file_range             -> php_cv_func_copy_file_range
+       - ac_cv_flush_io                    -> php_cv_have_flush_io
+       - ac_cv_func_getaddrinfo            -> php_cv_func_getaddrinfo
+       - ac_cv_have_broken_gcc_strlen_opt  -> php_cv_have_broken_gcc_strlen_opt
+       - ac_cv_have_pcre2_jit              -> php_cv_have_pcre2_jit
+       - ac_cv_syscall_shadow_stack_exists -> php_cv_have_shadow_stack_syscall
+       - ac_cv_time_r_type                 -> php_cv_time_r_type
+       - ac_cv_write_stdout                -> php_cv_have_write_stdout
+     and all other checks wrapped with their belonging cache variables (see *.m4
+     source files for details).
 
  c. Windows build system changes
    - The configure options --with-oci8-11g, --with-oci8-12c, --with-oci8-19,