This commit adds a patch to fix the case where gcc-6.1 emits an error
for string format literal that was a warning in previous versions of
gcc. The patch reference is
https://bugzilla.gnome.org/show_bug.cgi?id=761550
Also ran the package file through autopep8.
Package repositories now look like this:
top-level-dir/
repo.yaml
packages/
libelf/
package.py
mpich/
package.py
...
This leaves room at the top level for additional metadata, source,
per-repo configs, indexes, etc., and it makes it easy to see that
something is a spack repo (just look for repo.yaml and packages).
