Revert "[libc++] Only add dylib-related features when using the system's libc++"

This reverts r353319, which broke our internal CI.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@353321 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed