dropping support for GCC 4.8 and below
another point of discussion:
- with commit 8d55ef3e I broke compatibility with GCC 4.8.1 and below (which will once again show an internal segfault during compilation) for better readability esp. of typedefs for function pointers. (this could be circumvented by reverting the two using declarations inside the Tensor class)
- the warnings about deprecated function will only work with GCC >= 4.9
- GCC 4.9 was released 5 years ago next month. even debian oldstable uses it as the default - so the need to support even older compilers seems to be rather low imho.