package/fftw: enable OpenMP if the compiler has that

Unfortunately, this is limited to internal toolchains; we currently have
no way to know if an external toolchain has support for OpenMP.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: guillaume william brs <guillaume.bressaix@gmail.com>
Cc: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Tested-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Reviewed-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>