Fix RPATH information when only a genex is used as a link library.

As of commit 1da75022 (Don't include generator expressions in
old-style link handling., 2012-12-23), such entries are not
included in the LinkLibraries member. Generator expressions in
LinkLibraries are not processed anyway, so port to the new way
of getting link information.
Merge topic 'fix-genex-RPATH-handling' into next

dcb5907 Fix RPATH information when only a genex is used as a link library.