Added an AMD module registration.

Makefile

@@ -27,6 +27,7 @@ less:
 	@@cat ${HEADER} | sed s/@VERSION/${VERSION}/ > ${DIST}
 	@@echo "(function (window, undefined) {" >> ${DIST}
 	@@cat build/require.js\
+	      build/amd.js\
 	      build/ecma-5.js\
 	      ${SRC}/parser.js\
 	      ${SRC}/functions.js\

build/amd.js

@@ -0,0 +1,6 @@
+// amd.js
+//
+// Define Less as an AMD module.
+if (typeof define === "function" && define.amd) {
+    define("less", [], function () { return less; } );
+}