Revert "AC_CACHE_SAVE: treat x: as an absolute path as well"

This reverts commit 9d0402f4807bea7c00b518a7fa2b551a384b5906.

This is a redundant case. Is it there actually to use x: only ?
Revert "_AS_LN_S_PREPARE: default to 'cp -pR' on OS/2"

This reverts commit 0b60dcaebb9f53fdda08568b140f21be23b4e639.

Do not force to copy unconditionally. Respect user's choice.