Xcode: Fix static library creation for Xcode 6 (#15038)

Xcode 6 introduced an 'OTHER_LIBTOOLFLAGS' setting for the "Other
Librarian Flags" of a static library.  Now 'OTHER_LDFLAGS' are ignored.
Teach the Xcode generator to choose the correct name for the build
setting based on the type of target and the version of Xcode.

Inspired-by: Jamie Kirkpatrick <jkp@spotify.com>
Merge topic 'xcode-6-librarian-flags' into next

608cf814 Xcode: Fix static library creation for Xcode 6 (#15038)