All frontends will have to include <libsigrok/libsigrok.h> from now on.
This header includes proto.h and version.h, both installed from the
distribution into $INCLUDE/libsigrok/ as well.
The only dynamically changed header is now version.h, which has both
libsigrok and libtool compile-time versions in it.
This will come back in some form or another later, but for now
don't clutter the API with non-working stuff. Removing stuff from APIs
is not possible without breaking the API, adding stuff later is simpler.