Update 'morse_types.h' header to allow its use from C++ code

Add option to allow users' applications to redefine C complex types to C++ complex types (which are binary compatible with C complex types)

The library Fabulous ( git clone https://scm.gforge.inria.fr/anonscm/git/ib-bgmres-dr/ib-bgmres-dr.git --branch develop fabulous ) use Chameleon and is written in C++. Thus, it needs that chameleon header work with C++ complex types (in the same way it is done in LAPACKE header: see https://gitlab.inria.fr/solverstack/chameleon/blob/master/coreblas/include/lapacke_config.h#L91)

Limitations of this commit is that the codebase that would be using it not very wide yet (probably just Fabulous for now).
Therefore, this needs thorough reviewing and suggestions.

"Limitations of this commit is that the codebase that would be using it not very wide yet (probably just Fabulous for now)."

It would be great to have another C++ code using it, or at least the view of another adavanced C++ user. @sylvand could you give us your insight on it please?

Hum... unfortunately, I dont have much experience in "C library used in c++ code": with hmat, we do it the other way, and all the scalar S/D/C/Z arguments are passed as "void*", so we dont have to expose our complex types. Maybe someone could add in chameleon a small c++ test routine ?

OK: the C tests pass. The C++ interface has been validated through Fabulous. The code looks good. I suggest to merge and set up a todo for a small c++ unitary test within Chameleon.

merged

mentioned in commit cf6d3f4f