package/gnuradio: add optional support for orc

When orc was compiled before, gnuradio will use it as optional
dependency:

$ output/host/usr/bin/x86_64-linux-readelf -a output/target/usr/bin/gnuradio-config-info | grep NEEDED
[...]
 0x0000000000000001 (NEEDED)             Shared library: [liborc-0.4.so.0]
[...]

Apparently there is no option provided by the build system to
en-/disable support for orc:
https://github.com/gnuradio/volk/blob/41b4df055b2c8d4f2c23ad2bdc60c0b47b4f0dd8/lib/CMakeLists.txt#L449



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Reviewed-by: "Gwenhael Goavec-Merou" <gwenhael.goavec-merou@trabucayre.com>
Tested-by: "Gwenhael Goavec-Merou" <gwenhael.goavec-merou@trabucayre.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>