Don't use install(1) for the library symlinks in the build directory

It appears this was changed from ln to use install in rS245752. I noticed
this because my buildenv was setting INSTALL=install -U -M METALOG
and then these links fail to be created with the following error:
install: open METALOG: Permission denied

Reviewed By: brooks
Differential Revision: https://reviews.freebsd.org/D26618