Minor fixes for non-standard features (MSVC preliminary)

[roccoarpa]

This pull contains some minor fixes, mainly focused to the preliminary introduction of MSVC as official compiler of bitpit. Main contents are:

• Avoid using Variable Array Length (VLA) in static allocation of c-arrays. This feature is non-standard for all c-compilers. The basic rule (shared by all c compilers) is to allocate c-array with a const array-size. If the array-size is not known, switch to dynamic allocation and party's over. VLA let GNU and LLVM tolerate this, MSVC is inflexible and return all compile errors.
• Use a unique interface to call lapack stuffs, that is lapacke, since it's the official c-interface to lapack and widely portable.
• Some datatype casting (size_t to long) are safe into linux context, not into windows context (long is a 4-byte regular int under windows.)
• other miscellaneous fixes

[roccoarpa]

I've done the requested changes. Everything compiles fine and tests work. Asking again the review.

[andrea-iob]

Merged with a couple of cosmetic changes.