msvc: check if _MSC_VER is defined (#88)

double-conversion/double-conversion.cc

diff --git a/double-conversion/double-conversion.cc b/double-conversion/double-conversion.cc
index ecd1a5e..854f4b6 100644
--- a/double-conversion/double-conversion.cc
+++ b/double-conversion/double-conversion.cc
@@ -529,7 +529,7 @@
 // because it constant-propagated the radix and concluded that the last
 // condition was always true. By moving it into a separate function the
 // compiler wouldn't warn anymore.
-#if _MSC_VER
+#ifdef _MSC_VER
 #pragma optimize("",off)
 static bool IsDecimalDigitForRadix(int c, int radix) {
   return '0' <= c && c <= '9' && (c - '0') < radix;