cmake: Ensure linked MKL libraries are consistent with octopus build options.

As discussed and settled in issue #1137 (closed), we want serial builds of Octopus to link to sequential libraries.

Ensure Octopus links to MKL libraries that are consistent with the Octopus build options. For dynamic linking (MKL_LINK dynamic), this can be controlled by setting MKL_MPI and MKL_THREADING, rather explicitly linking to the desired components:

target_link_libraries(Octopus_lib PRIVATE
   MKL::mkl_core
   MKL::mkl_intel_${MKL_INTERFACE})

if (OCTOPUS_MPI)
target_link_libraries(Octopus_lib PRIVATE
   MKL::mkl_scalapack_${MKL_INTERFACE}
   MKL::mkl_blacs_${MKL_MPI}_${MKL_INTERFACE})
endif ()

if (OCTOPUS_OpenMP)
   target_link_libraries(Octopus_lib PRIVATE MKL::mkl_intel_thread)
else ()
   target_link_libraries(Octopus_lib PRIVATE MKL::mkl_sequential)
endif ()

This closes issue #1193 (closed), and it might as well close issue #1137 (closed) as this discussion was settled.

One can verify the variables returned from find_package(MKL CONFIG) with:

get_cmake_property(all_vars VARIABLES)

foreach(var IN LISTS all_vars)
   if(var MATCHES "^MKL")
     message(STATUS "MKL variable: ${var} = ${${var}}")
   endif()
endforeach()