Jessie now has gcc 4.9 not 4.7.

At least for me, the current Dockerfile build fails since the base image can't find candidate for g++-4.7 and even the gcc-4.7 request here actually installs gcc-4.9 for me right now which causes the update-alternatives command to fail.

I didn't see any particular reason to use 4.7 mentioned although there may well be one.

After making this change, whole build runs smoothly for me.

(If it helps my host machine is OS X but docker should make that immaterial as far as I understand).

If this is not desirable overall, any hints on how to get it to build with 4.7 would be appreciated.