COMP: fix build with -std=c++11 when ITK/VXL initially configured wit…

…hout

SimpleITK forces the compiler option '-std=c++11' when built against
libc++, such as is the case with Mac OS 10.9 or higher. However, by
default Slicer (and ITK) are not configured to use C++11. Thus when
SimpleITK is built against a non-'std=c++11' ITK build, there is a
mismatch between `VCL_CONSTEXPR` and the `VCL_CAN_STATIC_CONST_INIT_*`
preprocessor options, leading to compiler errors in Slicer and on the
SimpleITK dashboard [1,2]. Specifically, for '-std=c++11' compiler,

`VCL_CONSTEXPR = constexpr` as introduced in [3], but
`VCL_CAN_STATIC_CONST_INIT_{FLOAT,INT}` is undefined, leading to

`VCL_STATIC_CONST_INIT_FLOAT_DECL = ""`

leading to preprocessed definitions for VNL traits such as:

```
template <>
class vnl_numeric_traits<float>
...
static constexpr float zero ;
```

This causes the mentioned build errors because `constexpr` expressions
must be immediately constructible. The proposed patch unifies these
options so that they agree when VXL is compiled under '-std=c++11'.

[1] http://slicer-devel.65872.n3.nabble.com/build-error-in-simpleitk-td4035854.html
[2] https://open.cdash.org/buildSummary.php?buildid=4170100
[3] http://review.source.kitware.com/#/c/20386/

Change-Id: If04f4d3d0483fd8186e5a1189d183a9f0bb200d8

Modules/ThirdParty/VNL/src/vxl/core/vxl_config.h.in

@@ -28,16 +28,6 @@
 # define VXL_BIG_ENDIAN    @VXL_BIG_ENDIAN@
 #endif
 
-/* 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
-#else
-# define VCL_CONSTEXPR const
-#endif
-
 /* we can't just use typedefs, because on systems where there are   */
 /* no 64bit integers we have to #define vxl_int_64 to `void' in     */
 /* order to catch illegitimate uses. However, typedefs are superior */

Modules/ThirdParty/VNL/src/vxl/vcl/vcl_config_compiler.h.in

@@ -86,6 +86,18 @@
 //----------------------------------------------------------------------
 // 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