Avoid saying constexpr for VS2013. (#834)

Author: BillyONeal

Release/include/cpprest/asyncrt_utils.h

@@ -359,7 +359,13 @@ namespace details
     inline bool __cdecl is_alnum(const unsigned char uch) CPPREST_NOEXCEPT
     {   // test if uch is an alnum character
         // special casing char to avoid branches
-        static constexpr bool is_alnum_table[UCHAR_MAX + 1] =
+        static
+            #if !defined(_MSC_VER) || _MSC_VER >= 1900
+            constexpr
+            #else
+            const
+            #endif
+            bool is_alnum_table[UCHAR_MAX + 1] =
             {
             /*        X0 X1 X2 X3 X4 X5 X6 X7 X8 X9 XA XB XC XD XE XF */
             /* 0X */   0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,