Added ability to configure development dir [nodejs#21]

- Allows users to specify development directory using '--dev-dir' flag
- Added check for XDG_CACHE_HOME before HOME

lib/node-gyp.js

@@ -47,13 +47,15 @@ function Gyp () {
   var self = this
 
   // set the dir where node-gyp dev files get installed
-  // TODO: make this *more* configurable?
-  //       see: https://github.com/TooTallNate/node-gyp/issues/21
-  var homeDir = process.env.HOME || process.env.USERPROFILE
+  // user can specify this using the `dev-dir` flag or any of
+  // the below environment variables. Addresses [#21].
+  var homeDir = this.opts['dev-dir'] || process.env.XDG_CACHE_HOME
+    || process.env.HOME || process.env.USERPROFILE;
   if (!homeDir) {
     throw new Error(
-      "node-gyp requires that the user's home directory is specified " +
-      "in either of the environmental variables HOME or USERPROFILE"
+      "node-gyp requires that the user specify a development directory " +
+      "in either of the environmental variables XDG_CACHE_HOME, HOME, " +
+      "or USERPROFILE if the `dev-dir` flag is not used."
     );
   }
   this.devDir = path.resolve(homeDir, '.node-gyp')