Recommended C/C++ General Purpose DSP Libraries
Maybe a bit late, but since others might land here like me. 

The following are good signal processing libraries/frameworks:
  * https://www.gnuradio.org/ - lots of classic signal processing + cascading options
  * https://root.cern.ch/ - advanced statistical signal processing
Both provide generic facade, fall-back implementations as well as performance optimised sub-libraries (ie. fftw, atlas, LAPACK, gsl, VOLK, ...). 

Together with OpenCV, these probably cover 90++% of use-cases.