Update build-custom-glclient.sh

Using /bin/bash is almost always wrong.
Using /bin/sh is almost always correct
Using /usr/bin/env bash is better than /bin/bash

Consider cases where
a) /bin/bash is non-existent (most unixes)
b) /bin/bash is outdated (OSX)

I have not tested this with a POSIX shell but quickly skimmed it.

In addition get_sudo seems buggy: it can and should use "su" when sudo is not around.