Reordered FFTW fatal error; avoids using unset variables

Now variables that are set in FFTW detection are only used when the
variable that reports FFTW has been found is set (i.e. there has not
been a fatal error). Not sure why we haven't seen problems with this
so far, but we do see them on Power7 with cmake 2.8.8 when FFTWF is
not available.

Change-Id: I1a3e84002dda7f29f90eb532ab14ce51dfca6e5c