single precision fftw wanted

[Stephen Montgomery-Smith]

Yesterday I submitted a PR creating a port for gromacs.  I found a way 
to change some flags so that it also works with double precision fftw. 
It is at ports/71211.  It has WITHOUT_FFTW and WITH_FLOAT flags, etc, so 
when a single precision fftw port is created, the gromacs port can be 
easily modified.  (There is also a WITH_MPI flag, which I think also 
should be an option with fftw - this will allow people to use clusters 
of computers.)

To be honest I created this port because someone else asked me to - I 
have never actually used gromacs myself.  So if anyone else wants to 
take over the gromacs port (or write their own one) I am happy to pass 
it on.