Avoid assuming that FOLLY_HAVE_VLA is defined

Summary:
[Folly] Avoid assuming that `FOLLY_HAVE_VLA` is defined.

Treat undefined as meaning `0` rather than requiring it be defined to either `0` or `1`.

Closes #773.

Reviewed By: andrewjcg

Differential Revision: D7028174

fbshipit-source-id: 90c52a3c5b640776f728c70b69f0500749d7ab7f

folly/io/async/AsyncSocket.cpp

@@ -33,6 +33,12 @@
 #include <sys/types.h>
 #include <thread>
 
+#if FOLLY_HAVE_VLA
+#define FOLLY_HAVE_VLA_01 1
+#else
+#define FOLLY_HAVE_VLA_01 0
+#endif
+
 using std::string;
 using std::unique_ptr;
 
@@ -983,7 +989,7 @@ void AsyncSocket::writeChain(WriteCallback* callback, unique_ptr<IOBuf>&& buf,
     // suppress "warning: variable length array 'vec' is used [-Wvla]"
     FOLLY_PUSH_WARNING
     FOLLY_GCC_DISABLE_WARNING("-Wvla")
-    iovec vec[BOOST_PP_IF(FOLLY_HAVE_VLA, count, kSmallSizeMax)];
+    iovec vec[BOOST_PP_IF(FOLLY_HAVE_VLA_01, count, kSmallSizeMax)];
     FOLLY_POP_WARNING
 
     writeChainImpl(callback, vec, count, std::move(buf), flags);