package/fftw: prepare for splitting into multiple packages

package/fftw: prepare for splitting into multiple packages

In preparation for splitting into multiple packages, this commit
changes the fftw.mk package to use FFTW_COMMON_CONF_OPTS,
FFTW_COMMON_CONF_ENV and FFTW_COMMON_CFLAGS, which will be re-used by
the soon-to-be-introduced per-precision packages.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>