This warning doesn't seem accurate.  I'm not sure whether to change the warning you are giving, or to change the behaviour.

Currently as I understand it:

* If you are on windows and the HOMEDRIVE and HOMEPATH variables are not defined, print the warning.  `config_file` will remain null.
* `config_file += "/.haxelib"`, so `config_file = null+"/.haxelib"`;
* On [line 690](https://github.com/m22spencer/haxelib/blob/eed61260a40c6332fc66f4d36b2e3100dcac9365/src/tools/haxelib/Main.hx#L690), it will try to read "null/.haxelib" (and fail), so on windows it will attempt to fallback to `haxepath+REPNAME`, which is `haxepath+"lib"`

So our options are:

1. Change the warning to 'Warning: Environment variables %HOMEDRIVE% and %HOMEPATH% are missing, so we can't read the config file pointing to your haxelib repo.  We will default to use $haxepath/$REPNAME/ as the folder for your haxe repo.'  Or something similar (perhaps less long!)
2. Change the behaviour so that it attempts to read '$haxepath/.haxelib' as a config file. 

My vote would be for option 1.  And possibly cleaning up the code a little bit also. Trying to read "null/.haxelib", while it isn't lethal, is still not great.