Improving handling of optional processing blocks and linking libraries.
New options ENABLE_GN3S, ENABLE_RTLSDR, ENABLE_OPENCL, ENABLE_ARRAY and
ENABLE_GPERFTOOLS, all set by default to OFF. Users can enable that
features by 'cmake -DENABLE_XXX=ON ../ '. OpenCL is now not used by
default since it was giving problems in some platforms. Old variables
(RTSDR_DRIVER, GN3S_DRIVER, RAW_ARRAY_DRIVER and DISABLE_OPENCL) are
still honored and can be used in the same way, but the new ENABLE_XXX
are recommended for the sake of more uniform naming. Main CMakeFile.txt
has been rearranged, putting options first, then searching for the
required libraries, and then the optional ones. If Gperftools is enabled
and found, binaries are linked to the tcmalloc and profiler libraries,
and using the adequate flags. Fixed building in Mac OSX if Armadillo and
OpenBLAS were not installed in the system.