gtk: fix compilatione error with XCode 4.3/CLT

Add the flag --disable-visibility to configure, fixes compile errors
with Clang and llvm from CLT-4.3.  That flag means don't use ELF
visibility attributes.

Fixes Homebrew/homebrew#10283.
Closes Homebrew/homebrew#10287.

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

Formula/gtk+.rb

@@ -22,7 +22,8 @@ class Gtkx < Formula
                           "--disable-dependency-tracking",
                           "--prefix=#{prefix}",
                           "--disable-glibtest",
-                          "--disable-introspection"
+                          "--disable-introspection",
+                          "--disable-visibility"
     system "make install"
   end