Define FL_OVERRIDE unconditionally as 'override'

FLTK 1.5 requires at least C++11, hence we can always use 'override'.

To do: the FL_OVERRIDE macro can be replaced by 'override' everywhere
in the FLTK code. There are more than 2400 occurrences and this
would be a large commit. Therefore it's not done right now...

Note: user code may continue to use FL_OVERRIDE.

FL/fl_attr.h

@@ -1,7 +1,7 @@
 /*
  * Function attribute declarations for the Fast Light Tool Kit (FLTK).
  *
- * Copyright 1998-2024 by Bill Spitzak and others.
+ * Copyright 1998-2025 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
@@ -23,6 +23,21 @@
 #define _FL_fl_attr_h_
 
 
+/**
+  This macro makes it safe to use the C++11 keyword \c override with
+  older compilers.
+
+  Since FLTK 1.5.0 a C++11 capable compiler is required, hence we can safely
+  define FL_OVERRIDE unconditionally as 'override'.
+
+  This macro is kept for backwards compatibility with old (1.4.x) user code
+  that uses the FL_OVERRIDE macro.
+
+  We recommend to use 'override' explicitly if C++11 or higher is available.
+*/
+#define FL_OVERRIDE override
+
+
 /**
  This section lists macros for Doxygen documentation only. The next section
  will define the actual macros based on the compile used and based on the
@@ -39,11 +54,6 @@
  */
 #define __fl_attr(x)
 
-/**
-  This macro makes it safe to use the C++11 keyword \c override with
-  older compilers.
-*/
-#define FL_OVERRIDE override
 
 /**
  Enclosing a function or method in FL_DEPRECATED marks it as no longer
@@ -100,9 +110,6 @@
 #if defined(_MSC_VER)
 
 #if (_MSC_VER >= 1900) // Visual Studio 2015 (14.0)
-#ifndef FL_OVERRIDE
-#define FL_OVERRIDE override
-#endif
 #endif // Visual Studio 2015 (14.0)
 
 #if (_MSC_VER >= 1400) // Visual Studio 2005 (8.0)
@@ -138,9 +145,6 @@
 #endif // C++14
 
 #if (__cplusplus >= 201103L) // C++11
-#ifndef FL_OVERRIDE
-#define FL_OVERRIDE override
-#endif
 #endif // C+11
 
 #if (__cplusplus >= 199711L) // C++89
@@ -198,10 +202,6 @@
 #define __fl_attr(x)
 #endif
 
-#ifndef FL_OVERRIDE
-#define FL_OVERRIDE
-#endif
-
 #ifndef FL_DEPRECATED
 #define FL_DEPRECATED(msg, func) func
 #endif