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_ */

fluid/Fl_Type.h

@@ -1,16 +1,7 @@
 //
 // Widget type header file for the Fast Light Tool Kit (FLTK).
 //
-// Each object described by Fluid is one of these objects.  They
-// are all stored in a double-linked list.
-//
-// There is also a single "factory" instance of each type of this.
-// The method "make()" is called on this factory to create a new
-// instance of this object.  It could also have a "copy()" function,
-// but it was easier to implement this by using the file read/write
-// that is needed to save the setup anyways.
-//
-// Copyright 1998-2016 by Bill Spitzak and others.
+// 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
@@ -23,11 +14,22 @@
 //     https://www.fltk.org/bugs.php
 //
 
+// Each object described by Fluid is one of these objects.  They
+// are all stored in a double-linked list.
+//
+// There is also a single "factory" instance of each type of this.
+// The method "make()" is called on this factory to create a new
+// instance of this object.  It could also have a "copy()" function,
+// but it was easier to implement this by using the file read/write
+// that is needed to save the setup anyways.
+
 #include <FL/Fl_Widget.H>
 #include <FL/Fl_Menu.H>
 #include <FL/Fl_Plugin.H>
 #include "Fluid_Image.h"
 #include <FL/fl_draw.H>
+#include <FL/fl_attr.h>
+
 #include <stdarg.h>
 #include <stdlib.h>
 
@@ -859,12 +861,6 @@ void reveal_in_browser(Fl_Type*);
 int has_toplevel_function(const char *rtype, const char *sig);
 
 // file operations:
-#  ifdef __GNUC__
-#    define __fl_attr(x) __attribute__ (x)
-#  else
-#    define __fl_attr(x)
-#  endif // __GNUC__
-
 void write_word(const char *);
 void write_string(const char *,...) __fl_attr((__format__ (__printf__, 1, 2)));
 int write_file(const char *, int selected_only = 0);