Bugfix/gnutls (#3565)

* Fix nettle dependency

GnuTLS requires a nettle from the 2.7 series, won't work
with 3.x (so sayeth configure when it crashes...).

* Fix configure symlinking GNUmakefile to itself

I'm confused about how this ever worked, but....

As configure runs, it does something like this:

```
config.status: linking /home/hartzelg/tmp/spack/var/spack/stage/gnutls-3.3.9-uq2u4ecucncv2l3akran765aystyvkys/gnutls-3.3.9/GNUmakefile to GNUmakefile
```

and since that is the cwd the build crashes with:

```
==> 'make' '-j28'
make: GNUmakefile: Too many levels of symbolic links
make: stat: GNUmakefile: Too many levels of symbolic links
make: *** No rule to make target `GNUmakefile'.  Stop.
```

Since the comment in the configure script points out that the
GNUmakefile is "only for the maintainer" I've just commented out the
bit that leads to the symlinking.

Builds for me on CentOS 7.  (I do have a recent-ish set of auto-*
things in my patch via linuxbrew)

* Wordsmith

* Fix symbolic link problem properly

Autotools magic.  See #3565 for the details.

* Flake8 cleanup

* Add some headroom on the version number constraint

We really want < 3.0 but there's not a good way to express it.

We'd like to avoid having to update the file if they release 2.7.2.

This seems like a good compromise.

var/spack/repos/builtin/packages/gnutls/package.py

@@ -39,4 +39,8 @@ class Gnutls(AutotoolsPackage):
 
     version('3.3.9', 'ff61b77e39d09f1140ab5a9cf52c58b6')
 
-    depends_on("nettle")
+    # configure sez: Note that this version of gnutls doesn't support 
+    # nettle 3.0.
+    depends_on("nettle@:2.9")
+
+    build_directory = 'spack-build'