mk: Also build a static library
This combines the static libraries into one libud3tn.a that can be used for static linking. It shall be noted that we are using a "thin" archive (containing symlinks) for maximum compatibility - e.g., the ARM version of ar does not support conversion and we cannot always assume libtool to be present. The main objective is to build a library usable for linking it into uD3TN-NG.