Add support for Define_Switch with a callback in GNAT.Command_Line.

The callback is called for every instance of the switch found on the
command line. This make it possible to have full control over the
switch value and chain multiple actions if needed.

2018-05-30  Pascal Obry  <obry@adacore.com>

gcc/ada/

	* libgnat/g-comlin.ads (Value_Callback, Define_Switch): New.
	* libgnat/g-comlin.adb: Add corresponding implementation.

From-SVN: r260940