* configure.in: add major/minor/micro version number AC_SUBST

	* dbus/dbus-arch-deps.h.in (DBUS_MAJOR_VERSION,
	DBUS_MINOR_VERSION, DBUS_MICRO_VERSION, DBUS_VERSION_STRING,
	DBUS_VERSION): collection of macros to get version of library we
	are compiled against.

	* dbus/dbus-misc.c (dbus_get_version): new function, to get
	version of library we are linked against at runtime.