Configure scripts are designed to run under any bourne-compatible shell. bash is not the best choice, it's very slow. ash is much better to run configure scripts. It would be nice to have make.conf variable CONFIG_SHELL to let the user to specify environment variable CONFIG_SHELL to run configure scripts. On my installations CONFIG_SHELL=/bin/ash emegre blah is much faster than just "emerge blah".
Afaik, configure invokes /bin/sh for it's scripts; so just symlink it to some other bourne compatable shell?
gentoo installation has /bin/sh as a symlink to bash. There is no need to change this symlink. But configure scripts can use CONFIG_SHELL environment variable to use another shell to run configure script and make. The idea is simple - if the variable is not defined in make.conf, then do nothing, if defined - export it before staring configure script.
i fail to see how this is any different from exporting CONFIG_SHELL in your own environment
I propose just more convinient way to export the variable.