File _multibuild of Package fftw3

<multibuild>
  <package>standard</package>
  <package>gnu-hpc</package>
  <package>gnu-openmpi-hpc</package>
  <package>gnu-openmpi2-hpc</package>
  <package>gnu-openmpi3-hpc</package>
  <package>gnu-openmpi4-hpc</package>
  <package>gnu-mvapich2-hpc</package>
  <package>gnu-mpich-hpc</package>
</multibuild>