* Several backends supported, including UMESIMD and VCL.
  They can be selected by adding the env variable
  VECTOR_BACKEND. "none" is also supported, which will generate
  only the scalar code.
* UMESIMD supports various architectures, and is more fitting
  for this project than VCL as default.
* The following compiler options have been added:
* STATIC_VECTOR_WIDTH allows for a concrete vector width to be
  generated. STATIC_VECTOR_WIDTH=1 is supported with UMESIMD, as
  a "vector emulation". Regardless, running with -s 1 switches to
  the scalar version of the code always.
* PRECISION allows to change the generated code precision to
  single precision (PRECISION=single).
* TUNE will be automatically populated according to the
  compiler / machine under compilation. If it is manually set,
  it will not be overwritten.