Support cross compiling.

This should allow cross compiling with OpenELEC (not tested) just call configure with 
```./configure --enable-raspberrypi --host=$TARGET_NAME --cross-prefix=$TARGET_PREFIX```
As long as I understand their build system correctly.

I have not really tested cross compiling, because I don't have the necessary libraries cross-compiled. I have the Pi libraries and the script dies at freetype, so everything else should work.

A few other minor changes included in this changeset.