Fix compilation when using SDL < 2.0.14 (needed for SDL_OpenURL).

Abstract SDL functionality into SDL-specific class, away from Dialog class.

src/common/MediaFactory.hxx

@@ -181,6 +181,15 @@ class MediaFactory
     #endif
     }
 
+    static bool openURL(const string& url)
+    {
+    #if defined(SDL_SUPPORT)
+      return SDLOpenURL(url);
+    #else
+      return false;
+    #endif
+    }
+
   private:
     // Following constructors and assignment operators not supported
     MediaFactory() = delete;

src/common/SDL_lib.hxx

@@ -54,4 +54,13 @@ static inline string SDLVersion()
   return buf.str();
 }
 
+static inline bool SDLOpenURL(const string& url)
+{
+#if SDL_VERSION_ATLEAST(2,0,14)
+  return SDL_OpenURL(url.c_str()) == 0;
+#else
+  return false;
+#endif
+}
+
 #endif  // SDL_LIB_HXX

src/gui/Dialog.cxx

@@ -36,6 +36,7 @@
 
 #include "Vec.hxx"
 #include "TIA.hxx"
+#include "MediaFactory.hxx"
 
 /*
  * TODO list
@@ -183,13 +184,8 @@ const string Dialog::getHelpURL()
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
 void Dialog::openHelp()
 {
-  if(hasHelp())
-  {
-    if(SDL_OpenURL(getHelpURL().c_str()))
-    {
-      cerr << "error opening URL " << getHelpURL() << endl;
-    }
-  }
+  if(hasHelp() && !MediaFactory::openURL(getHelpURL()))
+    cerr << "error opening URL " << getHelpURL() << endl;
 }
 
 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -