Fix the theme compat file require paths to be fully absolute so they don't...

Fix the theme compat file require paths to be fully absolute so they don't rely on a particular php path to work. Fixes #14315 for trunk props dougal.

git-svn-id: https://core.svn.wordpress.org/trunk@15433 1a063a9b-81f0-0310-95a4-ce76da25c4cd