graphviz: patch for ghostscript>=9.16 (Homebrew#49)

Ghostscript>=9.16 renames their error codes from the prefix `e_`
to `gs_error_`. This patch, based on
ellson/MOTHBALLED-graphviz@f97c86e,
checks if the `e_` prefixed codes have been defined and sets them
equal to their `gs_error_` variants if they exist.

graphviz/patch-gvloadimage_gs.c.diff

@@ -0,0 +1,26 @@
+diff --git a/plugin/gs/gvloadimage_gs.c b/plugin/gs/gvloadimage_gs.c
+index 62dd3c9..6d9ed3d 100644
+--- a/plugin/gs/gvloadimage_gs.c
++++ b/plugin/gs/gvloadimage_gs.c
+@@ -32,6 +32,21 @@
+ #include <ghostscript/ierrors.h>
+ #include <cairo/cairo.h>
+
++
++/** Ensure compatibility with Ghostscipt versions newer than 9.16 **/
++
++#ifndef e_VMerror
++#define e_VMerror gs_error_VMerror
++#endif
++
++#ifndef e_unregistered
++#define e_unregistered gs_error_unregistered
++#endif
++
++#ifndef e_invalidid
++#define e_invalidid gs_error_invalidid
++#endif
++
+ #ifdef WIN32
+ #define NUL_FILE "nul"
+ #else