Only define constant if it's not defined.

A bit of safety, as @JeffPyeBrook mentioned in #1053
Merge pull request #1069 from jbeales/fix-typo-1053-option-2

Only define constant if it's not defined.
Added missing return values to two functions, wpsc_cart_tax and wpsc_cart_need_to_recompute_shipping_quotes
Merge pull request #1073 from JeffPyeBrook/pr-missing-retrun-values-in-utility-functions

Added missing return values to two functions, wpsc_cart_tax and wpsc_cart_need_to_recompute_shipping_quotes

Resolves issue #1072