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

llvm-svn: 356633
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: f1d87f8b4c430d72e7ab07e13508336a7841c3e6
diff --git a/docs/BuildingLibcxx.rst b/docs/BuildingLibcxx.rst
index cb00b89..fb22a1d 100644
--- a/docs/BuildingLibcxx.rst
+++ b/docs/BuildingLibcxx.rst
@@ -206,6 +206,13 @@
   libraries that may be used in with other shared libraries that use different
   C++ library. We want to avoid avoid exporting any libc++ symbols in that case.
 
+.. option:: LIBCXX_ENABLE_FILESYSTEM:BOOL
+
+   **Default**: ``ON`` except on Windows.
+
+   This option can be used to enable or disable the filesystem components on
+   platforms that may not support them. For example on Windows.
+
 .. _libc++experimental options:
 
 libc++experimental Specific Options