Increase the maximum number of formatting arguments on compilers without variadic templates to 15.

test/format-test.cc

@@ -1536,3 +1536,9 @@ void print_error(const char *file, int line, const char *format,
   fmt::print(format, args...);
 }
 #endif
+
+TEST(FormatTest, MaxArgs) {
+  EXPECT_EQ("0123456789abcde",
+	    fmt::format("{}{}{}{}{}{}{}{}{}{}{}{}{}{}{}",
+                        0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 'a', 'b', 'c', 'd', 'e'));
+}