ENH: define VCL_CONSTEXPR in configured file

We need a consistent definition of VCL_CONSTEXPR in
vcl/vcl_config_compiler.h.in for external projects
that may include this file.

vcl/vcl_config_compiler.h.in

@@ -87,6 +87,18 @@
 //----------------------------------------------------------------------
 // constant initializer issues.
 
+/* When using C++11 or greater, constexpr
+ * may be necessary for static const float initialization
+ * and is benificial in other cases where
+ * a value can be constant. */
+#if  __cplusplus >= 201103L
+# define VCL_CONSTEXPR constexpr
+# define VCL_CAN_STATIC_CONST_INIT_INT 1
+# define VCL_CAN_STATIC_CONST_INIT_FLOAT 1
+#else
+# define VCL_CONSTEXPR const
+#endif
+
 //: VCL_STATIC_CONST_INIT_INT_DECL(x)
 //
 // ANSI allows