Automatic building of Armadillo 3.x if gcc 4.6 was used is no longer
needed, so always download the latest version. Added a flag to compile
it with the same compiler than gnss-sdr is being built.
Recommend libopenblas-dev package to Ubuntu/Debian users
Updating Armadillo version to 4.300.8
Added requirement of GCC 4.7 or newer