cling (not clang > 7) : Has issues with the templated "vector_size" when using the GCC vector extensions.

Apparently cling is based on an old llvm clang so suffers from

• https://bugs.llvm.org/show_bug.cgi?id=16986
• https://bugs.llvm.org/show_bug.cgi?id=15730

For now cling should use the fallback.

Mentioning @mmuskinj as he has one case that there is also a dict created for the class he tried to modify https://its.cern.ch/jira/projects/ATLASSIM/issues/ATLASSIM-4753 and including vec.h makes cling fail on parsing the first alias . The fallback is obviously fine for cling

#if HAVE_VECTOR_SIZE_ATTRIBUTE

/// Define a nice alias for a built-in vectorized type.
template <typename T, size_t N>
using vec  __attribute__ ((vector_size(N*sizeof(T)))) = T;

#else

/// Define alias for the vectorized fallback type.
template <typename T, size_t N>
using vec = vec_fb<T, N>;

#endif