Fix & make linux conditionals uniform (#4278)

The source used a hodge-podge of different combinations of different
macros to check for linux: 'linux', '__linux', '__linux__'.

As '__linux__' is standard (Posix), and the others are not, the source
now uniformly uses __linux__. If either linux or __linux are defined,
it is made sure that __linux__ is defined as well.

src/guiChatConsole.cpp

@@ -630,7 +630,7 @@ bool GUIChatConsole::OnEvent(const SEvent& event)
 		}
 		else if(event.KeyInput.Char != 0 && !event.KeyInput.Control)
 		{
-			#if (defined(linux) || defined(__linux))
+			#if (defined(__linux__))
 				wchar_t wc = L'_';
 				mbtowc( &wc, (char *) &event.KeyInput.Char, sizeof(event.KeyInput.Char) );
 				prompt.input(wc);