Support for multiple and cleaner build commands

Closes #69

Adds a new syntax for the `build = ` command:

```
build = [
    ["./configure"],
    ["make", "lib/libovr.a"],
    ["/bin/mkdir", "target"],
    ["/bin/cp", "lib/libovr.a", "target/"]
]
```

The current syntax `build = "make"` still works.

`build = [ "a", "b" ]` is forbidden because it is ambiguous.
You also may not need the `ref` directives either.