Fix the CMake build on Windows when using PThreads

Summary: I broke things, so now I have to fix them.

Reviewed By: yfeldblum

Differential Revision: D4981674

fbshipit-source-id: 7df4d78cf4f984df158051a3a8b277a0bcad583b

CMake/folly-config.h.cmake

@@ -16,7 +16,7 @@
 
 #pragma once
 
-#cmakedefine FOLLY_HAVE_PTHREAD
+#cmakedefine FOLLY_HAVE_PTHREAD 1
 
 #define FOLLY_HAVE_LIBGFLAGS 1
 #define FOLLY_UNUSUAL_GFLAGS_NAMESPACE 1

folly/ThreadName.cpp

@@ -58,7 +58,7 @@ bool canSetOtherThreadName() {
 }
 
 bool setThreadName(std::thread::id tid, StringPiece name) {
-#if !FOLLY_HAVE_PTHREAD
+#if !FOLLY_HAVE_PTHREAD || _WIN32
   return false;
 #else
   static_assert(