111_contrib_FFTW_alternatives

In paper mentioned by https://github.com/hpcugent/easybuild-easyblocks/issues/111 you may notice that multiple versions of FFTW are popular among HPC calculations at scale.

As per http://www.fftw.org/, v2.1.5 and v3.x APIs are not compatible; it seems that,
it is practically necessary to include both FFTW variations for most HPC sites,
perhaps along with some --float variations, as evidenced by builds like:
https://github.com/fgeorgatos/easybuild.experimental/blob/master/users/fgeorgatos/GT/n/NAMD/NAMD-2.9-goalf-1.1.0.eb
https://github.com/fgeorgatos/easybuild.experimental/blob/master/users/fgeorgatos/GT/g/GROMACS/GROMACS-4.5.5-goalf-1.1.0-FLOAT.eb

ps. 
if you can come forward with a naming standard as regards the namespace for such parameterization, this is a really good moment to drop in the discussion...
Also, the altversions should become somewhat more integrated in, say, eb v1.3