COMP: Add -Wno-undefined-var-template to CMAKE_CXX_FLAGS
COMP: Add -Wno-undefined-var-template to CMAKE_CXX_FLAGS

Only needed when building the template externally with a clang compiler.

Hides the following spurious warnings:

```cpp
Modules/Core/Common/include/itkNumericTraits.h:1083:36: warning:
instantiation of variable 'itk::NumericTraits<std::__1::complex<double> >::Zero'
required here, but no definition is available [-Wundefined-var-template]

  static Self ZeroValue() { return Zero; }

Modules/Core/Common/include/itkNumericTraits.h:1083:36: note: add an
explicit instantiation declaration to suppress this warning if
'itk::NumericTraits<std::__1::complex<double> >::Zero' is explicitly
instantiated in another translation unit

  static Self ZeroValue() { return Zero; }
```

Fixes #120

phcerdan/ITKIsotropicWavelets,phcerdan/ITKIsotropicWavelets