Improve variant support for tup.

In my previous version for tup support, I assumed that the tup variant directory has the prefix `build-`. It is usually true if the variants are created with the `tup variant` command. However, it is possible to create variants manually. The only criterion is to put a `tup.config` file in a directory directly below the top level tup directory. I modified the script to find the variant directory this way, so that rtags can work more generally under tup.