Replace list of excluded platforms by name of chosen platform.

Since FL/platform.H defines USE_X11 for the X11 platform, it's better to target
the X11 platform by
  #include <FL/platform.H>
  #if USE_X11
rather than by
  #if !defined(_WIN32) && !defined(__APPLE__) && !defined(__ANDROID__)
that would have to grow with future platforms.

test/color_chooser.cxx

@@ -26,9 +26,7 @@
 
 #include <stdlib.h>
 #include <stdio.h>
-#if !defined(_WIN32) && !defined(__APPLE__) && !defined(__ANDROID__)
 #include "list_visuals.cxx"
-#endif
 
 int width = 100;
 int height = 100;
@@ -112,7 +110,7 @@ int main(int argc, char ** argv) {
            " - : default visual\n"
            " r : call Fl::visual(FL_RGB)\n"
            " c : call Fl::own_colormap()\n",argv[0]);
-#if !defined(_WIN32) && !defined(__APPLE__) && !defined(__ANDROID__)
+#if USE_X11
     printf(" # : use this visual with an empty colormap:\n");
     list_visuals();
 #endif
@@ -125,7 +123,7 @@ int main(int argc, char ** argv) {
     } else if (argv[i][0] == 'c') {
       Fl::own_colormap();
     } else if (argv[i][0] != '-') {
-#if !defined(_WIN32) && !defined(__APPLE__) && !defined(__ANDROID__)
+#if USE_X11
       int visid = atoi(argv[i]);
       fl_open_display();
       XVisualInfo templt; int num;

test/list_visuals.cxx

@@ -4,7 +4,7 @@
 // List all the visuals on the screen, and dumps anything interesting
 // about them to stdout.
 //
-// Does not use FLTK.
+// Does not use FLTK under X11.
 //
 // This file may be #included in another program to make a function to
 // call to list the visuals.  Fl.H must be included first to indicate this.
@@ -22,22 +22,19 @@
 //     https://www.fltk.org/bugs.php
 //
 
-#if defined(_WIN32) || defined(__APPLE__)
-#include <FL/Fl.H>
-#include <FL/fl_message.H>
+#ifndef Fl_H
+#  define NEED_MAIN 1 // when not included by another FLTK program
+#endif
 
-int main(int, char**) {
-  fl_alert("Currently, this program works only under X.");
-  return 1;
-}
+#include <FL/platform.H> // for USE_X11
 
-#else
+#if USE_X11
 
 #include <config.h>
 
 #define HAVE_MULTIBUF 0
 
-#ifndef Fl_H
+#ifdef NEED_MAIN
 
 #include <X11/Xlib.h>
 #include <X11/Xutil.h>
@@ -57,7 +54,7 @@ void fl_open_display() {
   fl_screen = DefaultScreen(fl_display);
 }
 
-#endif
+#endif // NEED_MAIN
 
 const char *ClassNames[] = {
   "StaticGray ",
@@ -216,14 +213,25 @@ void list_visuals() {
   if ( overlayInfo ) { XFree(overlayInfo); overlayInfo = 0; }
 }
 
-#endif
+#endif // USE_X11
 
-#ifndef Fl_H
-int main(int argc, char **argv) {
+#ifdef NEED_MAIN
+
+#  if ! USE_X11
+#    include <FL/fl_ask.H>
+#  endif
+
+int main(int argc, char** argv) {
+#  if USE_X11
   if (argc == 1);
   else if (argc == 2 && argv[1][0]!='-') dname = argv[1];
   else {fprintf(stderr,"usage: %s <display>\n",argv[0]); exit(1);}
   list_visuals();
   return 0;
+#  else
+  fl_alert("Currently, this program works only under X.");
+  return 1;
+#  endif // USE_X11
 }
-#endif
+
+#endif // NEED_MAIN

test/sudoku.cxx

@@ -437,7 +437,7 @@ void SudokuSound::play(char note) {
     Sleep(NOTE_DURATION);
   } else Beep(frequencies[note - 'A'], NOTE_DURATION);
 
-#else
+#elif USE_X11
 #  ifdef HAVE_ALSA_ASOUNDLIB_H
   if (handle) {
     // Use ALSA to play the sound...

test/tiled_image.cxx

@@ -33,9 +33,7 @@ void button_cb(Fl_Widget *,void *) {
 }
 
 #include <FL/platform.H>
-#if !defined(_WIN32) && !defined(__APPLE__)
 #include "list_visuals.cxx"
-#endif
 
 int visid = -1;
 int arg(int argc, char **argv, int &i) {
@@ -49,7 +47,7 @@ int arg(int argc, char **argv, int &i) {
 }
 
 int main(int argc, char **argv) {
-#if !defined(_WIN32) && !defined(__APPLE__)
+#if USE_X11
   int i = 1;
 
   Fl::args(argc,argv,i,arg);