autotools: Add autotools support for cross-compilation

I've been working to include a recipe in Yocto for pi-blaster on the Raspberry Pi. 

As a part of the recipe creation I modified the build to support autotools to make cross-compilation more straightforward. 

Perhaps this might be of interest? 

Signed-off-by: Alex J Lennon <ajlennon@dynamicdevices.co.uk>