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

Otherwise, when testing trunk libc++ on an older system, lit will think
that the dylib features are disabled. Ideally, we'd have a notion of
running the tests with/without a deployment target (or, equivalently,
a deployment target representing trunk where everything is as recent
as can be). Since we always have a deployment target right now (which
defaults to the current system), we only enable those features when
we're going to also be testing with the system libc++.

llvm-svn: 353319

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