Fixes to buildscripts

I am trying to run your proposed gdx-video extension, as we would be really interested in getting all issues resolved and the extension added to mainline libgdx. I am interested in getting the build-process working under linux 64-bits, but anything standard-ish that we can easily install in a VM would also be fine.

The changes in this PR allow me to build 32-bit and 64-bit linux native libraries (using Ubuntu Linux 12.04 64-bit), and I am currently working on a reliable way to build 32 and 64-bit versions of winpthreads.dll.

I may be able to help you further on ant integration -- I am by no means an expert, but if you can document your exact requirements, I would be happy to help.