opensp: fixup build on darwin by using older autoconf

Certainly feel free to find a better solution.
The usual ones were failing for me,
e.g. it used -std=gnu17 also for C++ (which fails).

pkgs/by-name/op/opensp/package.nix

@@ -7,6 +7,7 @@
   docbook_xml_dtd_412,
   docbook_xsl,
   autoconf,
+  autoconf269,
   automake,
   libtool,
   autoreconfHook,
@@ -61,7 +62,8 @@ stdenv.mkDerivation (finalAttrs: {
   ]
   # Clang 16 fails to build due to inappropriate definitions in the `config.h` generated by the
   # existing configure scripts. Regenerate them to make sure they detect its features correctly.
-  ++ lib.optional stdenv.cc.isClang autoreconfHook
+  # autoconf269: as autoconf 2.73 is trying to force C23 (-std=gnu23) which will fail, we use 2.69.
+  ++ lib.optionals stdenv.cc.isClang [ autoconf269 autoreconfHook ]
   ++ lib.optionals stdenv.hostPlatform.isCygwin [
     autoconf
     automake