We recently changed some aspects of the build system. Could you please open an issue on https://github.com/ethereum/solidity? Also your screenshot does not really show the error message, it should be a little further up.
I'm fairly confident that when I issue the following terminal command: geth --testnet --rpc --mine what is executed is the version of goethe that lives in my $GOPATH
This is a bash command so what program gets executed is determined by the $PATH environment var (only the go compiler is concerned with $GOPATH, not bash).
You can check what gets executed by ...
Download the binary file for your OS from here: https://geth.ethereum.org
Create a shell/bash file that contains instructions for starting up geth like in this example which I use for the ropsten testnet on Ubuntu. Make any changes as needed for the appropriate network you want to use as well as change/add any flags as needed. Be sure to change the ...