[libc++][CMake] Refactor how we link against system libraries

Instead of populating the global LIBCXX_LIBRARIES, we use the link-time
dependency management built into CMake to propagate link flags. This
leads to a cleaner and easier-to-follow build.

Reviewers: phosek, smeenai, EricWF

Subscribers: mgorny, christof, jkorous, dexonsmith, jfb, mstorsjo, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D60969

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@359571 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed