fftw: Refactor EXTRA_OECONF

The configure flags common to all three fftw recipes are moved into fftw.inc and
then each recipe just appends the extra flags it needs. This makes it easier to
read and modify options which affect all three recipes.

Signed-off-by: Paul Barker <paul@paulbarker.me.uk>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>