Make X11 font install dir the font search default (#2203)

* Make X11 font install dir the font search default

We install the X11 fonts into `/share/fonts` beneath the font-util
installation prefix, but that directory is not one of the places that
the font subsystem searches.

This commit makes the fontconfig package depend on the font-util
package, and then it makes

```python
spec['font-util'].prefix + "/share/fonts"
```

be the fontconfig default font location.

Before this change, plots drawn by R have bounding boxes where font
glyphs should be.  After this change fonts appear as expected.

* Enrich description string (trigger new CI run)

Improve the docstring for the package.

Also interested in the side effect of triggering another CI run
to see if the recent flake8 fix lets this PR run clean.

* Flake8 cleanups

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

@@ -26,7 +26,7 @@
 
 
 class Fontconfig(AutotoolsPackage):
-    """Fontconfig customizing font access"""
+    """Fontconfig is a library for configuring/customizing font access"""
     homepage = "http://www.freedesktop.org/wiki/Software/fontconfig/"
     url      = "http://www.freedesktop.org/software/fontconfig/release/fontconfig-2.11.1.tar.gz"
 
@@ -35,7 +35,12 @@ class Fontconfig(AutotoolsPackage):
     depends_on('freetype')
     depends_on('libxml2')
     depends_on('pkg-config', type='build')
+    depends_on('font-util', type='build')
 
     def configure_args(self):
-        args = ["--enable-libxml2", "--disable-docs"]
+        args = ["--prefix=%s" % prefix,
+                "--enable-libxml2",
+                "--disable-docs",
+                "--with-default-fonts=%s" %
+                spec['font-util'].prefix + "/share/fonts"]
         return args