graphviz: patch broken cdt.h header file

The cdt.h header duplicates the signatures of standard library functions
memcmp() and strcpy(), but those signatures don't match the real thing when
building with a C++ compiler, because they lack the no-throw declaration glibc
has. Simply dropping those redundant prototypes remedies the issue.

pkgs/tools/graphics/graphviz/default.nix

@@ -15,6 +15,8 @@ stdenv.mkDerivation rec {
       pango gd gts
     ] ++ stdenv.lib.optionals (xlibs != null) [ xlibs.xlibs xlibs.libXrender ];
 
+  patches = [ ./fix-broken-memcp-signature.patch ];
+
   configureFlags =
     [ "--with-pngincludedir=${libpng}/include"
       "--with-pnglibdir=${libpng}/lib"