Add warning if destination flag is ignored

[tobil4sk]

If the current target does not produce a file, the -Ddestination flag is currently ignored without warning.

This situation may occur commonly when 'default' points to another target, but doesn't produce a file itself. A warning will now be displayed:

Warning: Target 'default' does not output a file, so 'destination' has been ignored

[Simn]

I'm having a hard time making sense of that sentence, but I suppose that's because I lack context.

[tobil4sk]

Suppose you have a Build.xml like this:

<xml>

	<files id="main">
		<file name="main.c" />
	</files>

	<target id="main" output="main${EXE}" tool="linker" toolid="exe">
		<files id="main" />
	</target>

	<target id="default">
		<target id="main" />
	</target>

</xml>

The file has two targets, main which compiles a file, and default which just runs the main target (Haxe cpp gen produces a default and haxe target like this). -Ddestination is used to copy the output of a single target to a given file path.

Right now, if you use the default target (which hxcpp uses by default), the -Ddestination flag does not do anything since default does not produce an output file. The warning is meant to notify the user of this.