mention how to find out if libdeflate is used

git-svn-id: https://svn.r-project.org/R/trunk@88196 00db46b3-68df-0310-9c12-caf00c1e9a41

Author: ripley

src/library/base/man/memCompress.Rd

@@ -84,7 +84,10 @@ memDecompress(from,
   The headers and sources can be downloaded from
   \url{https://github.com/ebiggers/libdeflate} and pre-built versions
   are available for most Linux distributions.  It is used for binary
-  Windows distributions.
+  Windows and macOS distributions.
+
+  If it is used by an \R{} build and if so which version can be seen
+  from \code{\link{extSoftVersion}()}.
 }
 
 \value{