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

[phcerdan]

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

Hides the following spurious warnings:

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]

I removed the appveyor and circleci hooks from the github interface by the way. This will be the last commit with those hooks.

[phcerdan]

Green!