introduce the --define 'osx' as an alias for 'macosx'

compiler/options.nim

@@ -354,6 +354,7 @@ proc isDefined*(conf: ConfigRef; symbol: string): bool =
     of "msdos": result = conf.target.targetOS == osDos
     of "mswindows", "win32": result = conf.target.targetOS == osWindows
     of "macintosh": result = conf.target.targetOS in {osMacos, osMacosx}
+    of "osx": result = conf.target.targetOS == osMacosx
     of "sunos": result = conf.target.targetOS == osSolaris
     of "nintendoswitch":
       result = conf.target.targetOS == osNintendoSwitch