Documentation for MKL linking

[mferrero]

Installing TRIQS with mkl is not so easy. TRIQS is not detecting MKL and we will not try to include this feature. Nevertheless we could put a paragraph giving some hints on how to set the lapack/blas flags in order to use MKL. In particular it should emphasize that one needs static libraries for MKL otherwise one will run into RTD_GLOBAL-like problems with typical error messages:

*** libmkl_mc3.so *** failed with error : /xxx/lib/em64t/libmkl_mc3.so: undefined symbol: mkl_dft_commit_descriptor_s_c2c_md_omp