Cannot use --include path options

[alexkalderimis]

I get a reference error from a missing "os" variable, which is referenced here: https://github.com/marceloverdijk/lesscss-java/blob/master/src/main/resources/META-INF/lessc-rhino-1.7.0.js#L263-L273

This is clearly related to less/less.js#2005

[alexkalderimis]

I'm happy to fix this, but pointers and tips would be warmly welcomed