Separate logic for MKL linking from static option

[awvwgk]

Deletes the -Dstatic option, separates mkl, mkl-static (default) and mkl-rt as linear algebra backends.

la_backend	Description
mkl	link shared MKL libraries, produce dynamically linked binaries
mkl-static	link static MKL libraries, produce statically linked binaries
mkl-rt	link real time MKL library, produce dynamically linked binaries

Precompiled binaries will be provided with Intel 17 and mkl-static.

mkl-static is included for convenience and because it is the default for distribution of xtb.
Following setup is equivalent to mkl-static:

meson setup build -Dla_backend=mkl -Dfortran_link_args=-static

To link static binaries with other backends the -Dfortran_link_args=-static flag should be provided in the configuration step.

Fixes #237

[pierre-24]

... -Dla_backend=mkl works for me <3

[pierre-24]

(didn't you forget to upgrade the documentation, or is it going in the RTD one ?)

[awvwgk]

The static flag was a hidden feature for you so far. But the in detail description of all the possibilities to link the MKL will go to RTD rather than the meson guide here.