build: fix: configure fails if GOPATH environment variable not set

If the GOPATH enviroment variable was not set, go uses the default
GOPATH (~/go/).

The configure script was only checking if the GOPATH environment
is set. If it wasn't the script was failing.

Instead of checking if the GOPATH environment variable is set, check if
"go env GOPATH" returns a non-emtpy string.