Detect if lconv contains thousands_sep

(cherry picked from commit 96c28f748d)

test/util-test.cc

@@ -956,3 +956,17 @@ TEST(UtilTest, Conditional) {
   fmt::internal::Conditional<false, int, char>::type *pc = &c;
   (void)pc;
 }
+
+struct TestLConv {
+  char *thousands_sep;
+};
+
+struct EmptyLConv {};
+
+TEST(UtilTest, ThousandsSep) {
+  char foo[] = "foo";
+  TestLConv lc = {foo};
+  EXPECT_EQ("foo", fmt::internal::thousands_sep(&lc).to_string());
+  EmptyLConv empty_lc;
+  EXPECT_EQ("", fmt::internal::thousands_sep(&empty_lc));
+}