Changes

build: add --disable-multi-arfcn configure option

- add configure flag to disable Multi ARFCN support
- compile Channelizer, Synthesis, RadioInterfaceMulti and fft.c only when enabled
- guard Multi ARFCN-specific code with ENABLE_MULTI_ARFCN
- avoid requiring fftw3f when Multi ARFCN support is disabled

Change-Id: I2495c5b5c79f2fe74bef83902058e1d2207c1f3c