report empty GIT_DESCRIBE env variables, fixes issue #266

A `meta.yaml` file templated with `GIT_DESCRIBE` fails to parse the first time through, since the source is not available the first time it is parsed, and conda-build skips creating the environment variables if it doesn't find a `.git` directory in `SRC_DIR`. This pull request creates empty GIT_ environment variables if `.git` is not available, allowing `meta.yaml` to parse so the source can be provided for the second round of parsing (which already exists in conda-build).