Fix gdk-pixbuf module installation

When other packages install gdk-pixbuf modules, they get installed into
the gdk-pixbuf keg. To avoid this, we explicitly create the top level
lib directories for gdk-pixbuf and hack the pkgconfig file to point at
the top-level directories instead of the keg.

This results in other packages installing modules into the top-level
directory and as such they are "lost" and treated as unbrewed files, but
that is still better than losing them during upgrades of gdk-pixbuf
itself.

If we had a post-link hook, we could keep everything confined to its own
keg, linked into the top-level modules directory, and then run

    gdk-pixbuf-query-loaders --update-cache

ourselves. In the abscence of that, I think this is the best compromise.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/keg.rb

@@ -88,6 +88,7 @@ class Keg < Pathname
       # pkg-config database gets explicitly created
       when 'pkgconfig' then :mkpath
       # lib/language folders also get explicitly created
+      when /^gdk-pixbuf/ then :mkpath
       when 'ghc' then :mkpath
       when 'lua' then :mkpath
       when 'node' then :mkpath