Summary:
Building folly with cmake is only supported on Windows for now.  This fixes
cmake on non-Windows platforms to fail with a helpful message telling people to
use autoconf.  This message was in place before, but was after an MSVC version
check preventing it from appearing.

Reviewed By: meyering

Differential Revision: D6707328

fbshipit-source-id: a28a07ab0da41d605b11d93bba40f33520c5f57e