Fix how we check for server mode.

I can't tell exactly how the previous version worked, since it broke in
Haxe3 and switching back to Haxe2 to check is hard. But I've made it
simply pull the first line from the haxe default commandline output and
look for "(Haxe|haXe) Compiler X.X", which seems like it should work.
However I can't say that it does for certain because like I said I can't
switch back to check.