build-sys: avoid duplicate inclusion of log.c in tools

build-sys: avoid duplicate inclusion of log.c in tools

Before: shared code such as log.c was linked once into the public
libraries (where it is entirely hidden) and once into the various tools
which might use those libraries. This is suboptimal, as this way static
variables such as the maximum log level are instantiated twice in all
tools.

After: our build the public libraries a second time, as a convenience
libary, and link our tools against those. Hence all tools use only a
single instance of everything.