build: allow setting a custom GIT binary for transparent proxying

Some users can't run a bare 'git' command, due to need for a transparent
proxying solution such as 'tsocks'. This adds an argument to configure to
let users specify such a thing:

  ./configure --with-git="tsocks git"

The submodule script is also updated to give the user a hint about using this
flag, if we fail to checkout modules.

Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>