[hxp] Fix compatbility with haxe 3

[tobil4sk]

haxe 3 compatibility with hxp projects was broken in:: 22032ef

This is because haxe 3 doesn't support --main, only -main.

This gave errors like:

$ haxelib run lime build neko
Error: : unknown option '--main'.

[player-03]

According to the documentation, --main is correct, so I'd argue -main is less future-proof.

That said, -m is also officially supported. Does Haxe 3 recognize that?

[tobil4sk]

I'd argue -main is less future-proof.

Development versions of Haxe 5 still support -main, and there doesn't seem to be a push to get rid of it. For example, the haxe extension for vscode still generates templates using -main. I think if -main ever does stop working, haxe 3 support will probably be gone by then too.

That said, -m is also officially supported. Does Haxe 3 recognize that?

Seems like it doesn't:

$ ./haxe -m Main.hx
Error: : unknown option '-m'.
Haxe Compiler 3.4.7 - (C)2005-2017 Haxe Foundation
 Usage : haxe -main <class> [...]