Hi,

The emacs with native compile enabled will always rebuild all the
~/.emacs.d/eln-cache/VER/*.eln files if I moved/copied eln-cache
folder from
/home/userA/.emacs.d/eln-cache/ to
/home/userB/.emacs.d/eln-cache/.

It is caused by the function `comp-el-to-eln-rel-filename` calculating
the hash value from the absolute *.el filename to be the middle of the
eln file name.

The new variable `comp-el-to-eln-strip-prefix` in this patch will
allow moving the eln-cache/ folder without rebuilding *.eln in it.

Please help review this change, Thanks

Best Regards
Lin