maint: implement proper library versioning. * libcruft/Makefile.am (libcruft_current): New variable. (libcruft_revision): Likewise. (libcruft_age): Likewise. (libcruft_version_info): Likewise. (libcruft_la_LDFLAGS): Use -version-info option, not -release. * liboctave/Makefile.am (liboctave_current): New variable. (liboctave_revision): Likewise. (liboctave_age): Likewise. (liboctave_version_info): Likewise. (liboctave_la_LDFLAGS): Use -version-info option, not -release. * liboctinterp/Makefile.am (liboctinterp_current): New variable. (liboctinterp_revision): Likewise. (liboctinterp_age): Likewise. (liboctinterp_version_info): Likewise. (liboctinterp_la_LDFLAGS): Use -version-info option, not -release.