Do not download config scripts during libsodium autogen

libsodium's autogen.sh fetches config.guess/config.sub from
git.savannah.gnu.org with curl -sL but no -f, so a server error (e.g.
502) is silently written into build-aux/config.sub and configure later
dies with 'cannot run /bin/sh ./build-aux/config.sub'. The scripts
autoreconf installs are recent enough for every platform we build on,
so opt out of the download via DO_NOT_UPDATE_CONFIG_SCRIPTS to keep
the build hermetic.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: maennchen

c_src/Makefile

@@ -192,7 +192,7 @@ $(LIBSODIUM_CONFIGURE_FILE): $(LIBSODIUM_SRC_DIR)
 	$(verbose) if [ ! -f $(LIBSODIUM_CONFIGURE_FILE) ]; then \
 		(cd $(LIBSODIUM_SRC_DIR) \
 			&& chmod +x $(LIBSODIUM_AUTOGEN_FILE) \
-			&& $(LIBSODIUM_AUTOGEN_FILE)); \
+			&& DO_NOT_UPDATE_CONFIG_SCRIPTS=1 $(LIBSODIUM_AUTOGEN_FILE)); \
 	else \
 		true; \
 	fi