package/fftw: disable fortran

We do not have any fortran package and we do not build a fortran
compiler.

Still, an external toolchain may have a fortran compiler and that is
totally useless to us, since we anyway have no fortran package...

So, entirely disable fortran support; that will at least slightly speed
up the build in case an external toolchain has a fortran compiler.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[based on a patch by Guillaume]
Cc: guillaume william brs <guillaume.bressaix@gmail.com>
Cc: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Tested-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Reviewed-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>