buildroot

fftw: add compile precision option

fftw has options to select compile precision between single, long-double
and quad. These options are exclusives. This patch adds choice to select
precision option.

Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
[yann.morin.1998@free.fr: use --disable-XXX when not enabled; reword
 prompts (default is not 'none' but 'double')
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: guillaume william brs <guillaume.bressaix@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>