--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -626,7 +626,7 @@ cache_cf.o: cf_parser.cci
 
 # cf_gen builds the configuration files.
 cf_gen$(EXEEXT): $(cf_gen_SOURCES) $(cf_gen_DEPENDENCIES) cf_gen_defines.cci
-	$(BUILDCXX) $(BUILDCXXFLAGS) -o $@ $(srcdir)/cf_gen.cc -I$(srcdir) -I$(top_builddir)/include/ -I$(top_builddir)/src
+	$(BUILDCXX) -std=c++17 -o $@ $(srcdir)/cf_gen.cc -I$(srcdir) -I$(top_builddir)/include/ -I$(top_builddir)/src
 
 # squid.conf.default is built by cf_gen when making cf_parser.cci
 squid.conf.default squid.conf.documented: cf_parser.cci