improve support for vectorization in CMake

* the user can specify one of {none,AVX,AVX2,KNL} as
  targets for vectorization
* if modcc is available in PATH it will be used, which
  makes cross compilation easier
* a new flag USE_OPTIMIZED_KERNELS can be set to
  generate optimized kernels from the modcc compiler
  (equivalent to adding -O flag to modcc)