You can set options by setting things on a global LESS object before the script:
<!-- set options before less.js script -->
<script>
  less = {
    env: "development",
		async: false,
		fileAsync: false,
		poll: 1000,
		functions: {},
		dumpLineNumbers: "comments",
		relativeUrls: false,
		rootpath: ":/a.com/"
	};
</script>
<script src="less.js"></script>Type: String
Default: development
Environment to run may be either development or production
Type: Boolean
Default: false
Load imports asynchronously.
Type: Boolean
Default: false
Load imports asynchronously when in a page under a file protocol.
Type: Integer
Default: 1000
The amount of time (in milliseconds) between polls while in watch mode.
Type: object
User functions, keyed by name.
Type: String
Default: comments
Or "mediaQuery" or "all". TODO: need more explaination here.
Type: Boolean
Default: false
Optionally adjust URL's to be relative. When false, URL's are already relative to the entry less file.
Type: String
Default: false
A path to add on to the start of every URL resource.