cmake: define FOLLY_USE_LIBSTDCPP in folly-config.h

Summary:
This setting is already set by FollyConfigChecks.cmake, but was not
actually defined by folly-config.h

This setting is only checked by a few of the concurrency tests.

Reviewed By: igorsugak

Differential Revision: D10405959

fbshipit-source-id: 1602730e6adb7e05a82e65bdc5c367d105f59224

CMake/folly-config.h.cmake

@@ -36,6 +36,7 @@
 #cmakedefine FOLLY_HAVE_LIBGLOG 1
 
 #cmakedefine FOLLY_USE_JEMALLOC 1
+#cmakedefine FOLLY_USE_LIBSTDCPP 1
 
 #if __has_include(<features.h>)
 #include <features.h>