fix gcc <= 7.1 compile errors

test/scan.h

@@ -169,16 +169,24 @@ struct scan_handler : error_handler {
     scan_ctx_.advance_to(it + size);
   }
 
-  FMT_CONSTEXPR int on_arg_id() { return on_arg_id(next_arg_id_++); }
-  FMT_CONSTEXPR int on_arg_id(int id) {
+// GCC 7.1.x and below fail to compile if on_arg_id is declared constexpr
+#if FMT_GCC_VERSION && (FMT_GCC_VERSION < 720)
+#  define FMT_CONDITIONAL_CONSTEXPR
+#else
+#  define FMT_CONDITIONAL_CONSTEXPR FMT_CONSTEXPR
+#endif
+
+  FMT_CONDITIONAL_CONSTEXPR int on_arg_id() { return on_arg_id(next_arg_id_++); }
+  FMT_CONDITIONAL_CONSTEXPR int on_arg_id(int id) {
     if (id >= args_.size) on_error("argument index out of range");
     arg_ = args_.data[id];
     return id;
   }
-  FMT_CONSTEXPR int on_arg_id(string_view id) {
+  FMT_CONDITIONAL_CONSTEXPR int on_arg_id(string_view id) {
     if (id.data()) on_error("invalid format");
     return 0;
   }
+#undef FMT_CONDITIONAL_CONSTEXPR
 
   void on_replacement_field(int, const char*) {
     auto it = scan_ctx_.begin(), end = scan_ctx_.end();