Automake does not easily allow to use the same source in a libtool
library and in a program

In case of SSE/AVX/AVX2 it could happen that more than one kernel
(since some depend on other kernels, e.g. block 6 on block 4 and
block 2) were called

The issue why these test programs are simply not build
has to be investigated

Test

-with/without mpi
-with/without openmp
-with/without ftimings
-with/without redirect

all kernels

It turned out that if a CPU supports SSE the already existing
test for SSE assembly instructions always passes.
However, the compilation of gcc SSE intrinic instructions might
nevertheless fail if gcc is not called with one of the options
"-msse3", "-msse4" , "-msse4.1", "-msse4.2", "-mavx", or "-mavx2"!

Obviously gcc does still not consider SSE as a standard on X86_64
Intel CPUs.

An additional configure test has been introduced, which test for
gcc intrinsic sse instructions. If this test fails, the corresponding
kernels are switched off.

The C++ kernels can be written as C kernels, which simplifies the build
procedure

In order to increase type safty all ELPA2 kernels provide now
an interface. The interfaces for the C/C++ kernels are automatically
generated during the configure step