mysql: listen on 127.0.0.1 by default

It's generally not a good idea to allow remote connections to mysql.
Make it safe by default.

Closes #20090.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
superenv: Improve compiler selection

This fixes superenv on Xcode 3.2.6, and also fixes C++ software with
gcc-4.2 on all Xcodes.