package/fftw: break fftw-quad into a dedicated package

package/fftw: break fftw-quad 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 quad precision into a new package and:

- makes BR2_PACKAGE_FFTW_PRECISION_QUAD select BR2_PACKAGE_FFTW_QUAD
  to keep compatibility with packages that use
  BR2_PACKAGE_FFTW_PRECISION_QUAD. This option will be removed in a
  follow-up commit;

- makes fftw depend on fftw-quad when this package is enabled.

Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
[Yann/Thomas:
 - Force --disable-quad in FFTW_CONF_OPTS, just for the sake of
   clarity (fftw is no longer going to build the quad variant)
 - Use FFTW_QUAD_DL_SUBDIR to avoid downloading fftw multiple times
 - Minor reformatting tweaks in fftw-quad.mk
 - Do not deprecate BR2_PACKAGE_FFTW_PRECISION_QUAD and instead
   make it select BR2_PACKAGE_FFTW_QUAD, so that packages using
   BR2_PACKAGE_FFTW_PRECISION_QUAD continue to work.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
[Gwenhael/Yann]:
 - (BR2_i386 || BR2_x86_64) -> BR2_TOOLCHAIN_HAS_LIBQUADMATH
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>