The generic real double and single precision kernels are basically
identical, thus one can use the same template for them.

Since Fortran lacks proper templates, the template is produced
with preprocessor defines

Sometimes it might be desirable that only one of the specifc AVX
kernels (AVX, AVX2, and in the future AVX512) are build, even though the processor
supports all (or several) of these instructions.

The flag --disable-[avx | avx2 | avx512] allows to define which kernels
should NOT be build. The default is: what can be build will be build