Add syntax highlighting for autoconf variables

Add syntax highlighting for autoconf variable substitution (@VARIABLE@)
![screenshot](https://cloud.githubusercontent.com/assets/3471434/3999500/c6593816-2950-11e4-8017-202fdf7dd147.png)

What do you think about adding a `\\b` at the beginning and end of this pattern to make sure they are only parsed as strings across word boundaries?