Avoid multiple definition of macro __fl_attr()

This could happen if both FL/fl_ask.H and fluid/Fl_Type.h were
included in the same file.

FL/fl_ask.H

@@ -22,6 +22,7 @@
 #  define fl_ask_H
 
 #  include "Enumerations.H"
+#  include "fl_attr.h"
 class Fl_Widget;
 
 /** Different system beeps available.
@@ -35,14 +36,6 @@ enum Fl_Beep {
   FL_BEEP_NOTIFICATION  ///< Notification beep.
 };
 
-#  ifdef __GNUC__
-/* the GNUC-specific attribute appearing below in prototypes with a variable list of arguments
- helps detection of mismatches between format string and argument list at compilation time */
-#    define __fl_attr(x) __attribute__ (x)
-#  else
-#    define __fl_attr(x)
-#  endif // __GNUC__
-
 FL_EXPORT void fl_beep(int type = FL_BEEP_DEFAULT);
 FL_EXPORT void fl_message(const char *,...) __fl_attr((__format__ (__printf__, 1, 2)));
 FL_EXPORT void fl_alert(const char *,...) __fl_attr((__format__ (__printf__, 1, 2)));

FL/fl_attr.h

@@ -0,0 +1,34 @@
+/*
+ * Function attribute declarations for the Fast Light Tool Kit (FLTK).
+ *
+ * Copyright 1998-2021 by Bill Spitzak and others.
+ *
+ * This library is free software. Distribution and use rights are outlined in
+ * the file "COPYING" which should have been included with this file.  If this
+ * file is missing or damaged, see the license at:
+ *
+ *     https://www.fltk.org/COPYING.php
+ *
+ * Please see the following page on how to report bugs and issues:
+ *
+ *     https://www.fltk.org/bugs.php
+ */
+
+#ifndef _FL_fl_attr_h_
+#define _FL_fl_attr_h_
+
+/*
+  The GNUC-specific attribute appearing below in prototypes with a variable
+  list of arguments helps detection of mismatches between format string and
+  argument list at compilation time.
+
+  Examples: see fl_ask.H
+*/
+
+#ifdef __GNUC__
+#  define __fl_attr(x) __attribute__ (x)
+#else
+#  define __fl_attr(x)
+#endif
+
+#endif /* !_FL_fl_attr_h_ */