Setup fixes for OSX

- Use a more robust way of determining the location of the setup script
- Style fix: make INCLUDE path constant

bin/setup

@@ -3,11 +3,15 @@
 #
 # Prelude
 #
-INCLUDE=$(dirname $(readlink -f $0))
-. $INCLUDE/setup_tools
-
 set -euo pipefail
 IFS=$'\n\t'
+# Beware of CDPATH gotchas causing cd not to work correctly when a user 
+# has set this in their environment
+# https://bosker.wordpress.com/2012/02/12/bash-scripters-beware-of-the-cdpath/
+unset CDPATH
+
+readonly INCLUDE="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
+. "${INCLUDE}"/setup_tools
 
 #
 # Main