update version in configure.ac

the (new) configure.ac has a notion of the current version, which is passed via the AC_INIT() command.
in Pd-0.43.1 this version is still set to 0.43.0, which looks a bit weird when compiling (as the wrong version shows up)

given that this version is not really used anywhere, the attached patch is purely cosmetic (and outdated whenever applied :-)

nevertheless it would be nice if the versions could be kept in sync.
it would be even nicer if somebody could provide a bit of code that fills the AC_INIT() version into m_pd.h