More build script fixes and tweaks

[DilumAluthge]

1. Explicitly use Bash, instead of worrying about being compatible with any shell.
2. Use ${foo:?} whenever possible, to make sure that variables are defined and not empty.
3. At the beginning of the script, make sure all dependencies exist, before we start running any commands.

Also includes the macOS sed fix from #8.

[DilumAluthge]

Bump @IanButterworth

[IanButterworth]

Go for it, if you've tested this

[DilumAluthge]

I need to do some more testing. For now, I'll cut [email protected] off of your #8 branch.