[patch] performance improvement in gr-osmosdr (hackrf)

Hi, I have submitted a pull request on Github[1] which improves the performance of the hackrf source. With this patch the conversion int8_t -> float is done in realtime instead of looking up values in the "_lut" lookup table. In this way the compiler is able to generate AVX/SSE code (-O3 -march=native) to perform the conversion. In [2] you can find a benchmark to show the differences. Using -Ofast (or -O3) and -march=native I get ~2.8x using an Intel i7 (4th gen) and 3.1x on a Core i5 (6th gen). Gqrx with the patched library can play a WFM radio without any interruption even with just 3 buffers (option="hackrf=0,buffers=3"). Best regards, Alain [1] https://github.com/osmocom/gr-osmosdr/pull/14 [2] https://gist.github.com/carpikes/cad029c338605f70d9f687aeee447db4