[libc++] Add a per-target flag to include the generated config_site

This allows propagating the include automatically to targets that
depend on one of the libc++ targets such as the benchmarks. Note
that the GoogleBenchmark build itself still needs to manually specify
the -include, since I don't know of any way to have an external project
link against one of the libc++ targets (which would propagate the -include

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@373631 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed