AX_PTRDIFF_ALIASES_INT: the check should be more reliable now

* It was failing on OpenSUSE Tumbleweed i586 on OBS,
  resulting in duplicate symbols and build errors.
  This was not reproducible with 32-bit builds on OpenSUSE Tumbleweed x86_64.
* Instead of hoping for a compiler warning to be treated as an error,
  we now use a static assertion with a _Generic expression.
* The scintilla submodule has also been updated since Neil was asking to
  update the documentation as well.

1 files changed, 0 insertions, 0 deletions

diff --git a/contrib/scintilla b/contrib/scintilla
-Subproject 333dd3153850d05a219731dee0b899155442b19
+Subproject db63b6316586873ae1fd13318787dca58602664