package/fftw: break fftw-long-double into a dedicated package

package/fftw: break fftw-long-double into a dedicated package

fftw's library name depends on the precision option. Consequently,
it's possible to install multiple flavor on the same target.

This patch breaks fftw long double precision into a new package and:

- makes BR2_PACKAGE_FFTW_PRECISION_LONG_DOUBLE select
  BR2_PACKAGE_FFTW_LONG_DOUBLE to keep compatibility with packages
  that use BR2_PACKAGE_FFTW_PRECISION_LONG_DOUBLE. This option will be
  removed in a follow-up commit;

- makes fftw depend on fftw-long-double when this package is enabled.

Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
[Yann/Thomas:
 - Force --disable-long-double in FFTW_CONF_OPTS, just for the sake of
   clarity (fftw is no longer going to build the long double variant)
 - Use FFTW_LONG_DOUBLE_DL_SUBDIR to avoid downloading fftw multiple times
 - Minor reformatting tweaks in fftw-long-double.mk
 - Do not deprecate BR2_PACKAGE_FFTW_PRECISION_LONG_DOUBLE and instead
   make it select BR2_PACKAGE_FFTW_LONG_DOUBLE, so that packages using
   BR2_PACKAGE_FFTW_PRECISION_LONG_DOUBLE continue to work.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Tested-by:  "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>