libpcap: needs newer bison/flex

Fixes #574.

Library/Formula/libpcap.rb

@@ -14,10 +14,10 @@ class Libpcap < Formula
 
   keg_only :provided_by_osx
 
-  unless OS.mac?
-    depends_on "bison" => :build
-    depends_on "flex" => :build
-  end
+  # System versions are too old on older OS Xs
+  # https://github.com/mistydemeo/tigerbrew/issues/574
+  depends_on "bison" => :build
+  depends_on "flex" => :build
 
   def install
     system "./configure", "--prefix=#{prefix}",