We ran into problems with precompiled protoc binaries not working on
some systems because of the dependence on a dynamically linked
libatomic. However, I found that linking against libatomic is not
actually necessary in our protoc build environment. Inside configure.ac
there's a test which figures out if we need to pass -latomic, and it
does this by attempting to build a test binary. That binary was failing
to build, but it turns out this was because we need to pass -std=c++11,
not -latomic.

This fixes #5875.