Add add'l output formats for graphviz (#4431)

* Add add'l output formats for graphviz

Add support for additional output formats to graphviz, including gif,
jpg, pdf, and png.

Graphviz calls its pango+cairo option *pangocairo* so I followed suit.

Libgd was missing jpeg/jpg support.  None of the other supported
formats are conditionalized and there is no --with/--without support,
so I followed suit.

* Fix ghostscript plugin

When I installed thusly:

    spack install graphviz+pangocairo+libgd^cairo+X^pango+X

the ghostscript plugin tripped over some variable names that had
changes in `gs@9.18:`.  This fixes them.

* Remove wayward import of tty

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

@@ -54,5 +54,6 @@ class Libgd(AutotoolsPackage):
 
     depends_on('libiconv')
     depends_on('libpng')
+    depends_on('jpeg')
     depends_on('libtiff')
     depends_on('fontconfig')