scintilla: use AX_PTRDIFF_ALIASES_INT to check whether ptrdiff_t aliases int

* Scintilla had build problems if ptrdiff_t doesn't alias int if it has the
  same storage size (e.g. on NetBSD 10 (armv6)).
* Unfortunately, Neil Hodgson refused to merge my previous Scintilla patches that
  would have fixed the issue at the core.
  See https://groups.google.com/g/scintilla-interest/c/STAv6LgLyCo
* He only agreed to introducing a separate flag to work around the issue
  (`-DPTRDIFF_DOESNT_ALIAS_INT`).
* In order to continue to support all standard C/C++ conforming platforms
  we therefore need a compile-time check to check for the aliasability
  of ptrdiff_t and int.

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