Add no-op fontconfig files for Microsoft fonts.

With these fontconfig files it becomes possible for other packages to
configure font aliases for the Microsoft fonts. If both, for example,
vista-fonts and some package providing an alias for Cambria are
installed then the user may control whether or not the alias is active
by manipulating the package priorities.

pkgs/data/fonts/corefonts/no-op.conf

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
+<fontconfig>
+  <!-- This configuation is intentionally left empty in order to
+       override any other font package that may wish to set up an
+       alias for the Microsoft @fontname@ font. If you actually do
+       want to have the alias then please change the priority of that
+       package; see the Nix manual page for nix-env for details. -->
+</fontconfig>