target_link_libraries: Allow repeated use of LINK_{PRIVATE,PUBLIC} keywords.

This has not been allowed since they were introduced in
commit 91438222 (target_link_libraries: Add LINK_(PUBLIC|PRIVATE)
options, 2011-10-07), but allowing this form makes it more compatible
with the newer PUBLIC and PRIVATE keywords.
Merge topic 'allow-repeated-LINK-keywords' into next

dede273 target_link_libraries: Allow repeated use of LINK_{PRIVATE,PUBLIC} keywords.