.gitmodules: use relative submodule URLs.

The previously configured URLs were project homepages [1], not Git
repositories [2].  Rather than hardcode a full URL here, it's probably
better to just use a relative URL.  `git submodule` interprets URLs
from .gitmodules relative to the superproject's origin, so

  $ git clone git://github.com/inducer/pycuda.git

sets up

  $ git config remote.origin.url
  git://github.com/inducer/pycuda.git

so the new .gitmodules URLs resolve to

  git://github.com/inducer/bpl-subset.git
  git://github.com/inducer/compyte.git

[1]: http://github.com/inducer/bpl-subset
[2]: https://github.com/inducer/bpl-subset.git
     git://github.com/inducer/bpl-subset.git