Allow disabling of filesystem library.

Summary: Filesystem doesn't work on Windows, so we need a mechanism to turn it off for the time being.

Reviewers: ldionne, serge-sans-paille, EricWF

Reviewed By: EricWF

Subscribers: mstorsjo, mgorny, christof, jdoerfert, libcxx-commits

Differential Revision: https://reviews.llvm.org/D59619

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@356633 91177308-0d34-0410-b5e6-96231b3b80d8
8 files changed