cmd/go: check for SWIG version that is too old to use
cmd/go: check for SWIG version that is too old to use

Fixes issue 7983.

LGTM=crawshaw
R=golang-codereviews, crawshaw
CC=golang-codereviews, rsc
https://codereview.appspot.com/96540044

firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,wesolows/golang,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang