As per summary, cp -a won't work on BSD and Darwin userlands. The attached patch uses cp -pR instead and allows it to be installed fine. Thanks, Diego
Created attachment 66588 [details, diff] docbook-xsl-stylesheet-ebuild.patch
Maybe -pPR is a better alternative to -a on BSD/Darwin? (prevents from following symlinks)
"The less, the best" usually, I haven't found symlinks there, and I don't think there will be in such a package. But yeah if you want to be completely safe, -pPR is the full replacement of -a.
*** This bug has been marked as a duplicate of 103487 ***