build: no -Wrestrict warning under gcc 12 (from dev e727ce1d248d)

extended_latin_symbol_pattern() assigned a string literal to a std::string
inside its loop, for which gcc 12 reports a bogus memcpy overlap at -O3.  The
separator is a const char* now.  No behaviour change.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

README.md

@@ -51,7 +51,7 @@ are regenerated on each release — patches cannot be merged directly.
 Report problems (or send patches) to the author; accepted changes are
 applied to the development tree and appear in a following snapshot.
 
-This snapshot was assembled from development commit `2e453744f5e9`.
+This snapshot was assembled from development commit `e727ce1d248d`.
 
 ## License