Choose whether to use MSVC based on MSVC env.

I had to setup a new box to build JNA and it was as painful as usual :) 

I've updated the instructions thoroughly, separating 32-bit Windows and 64-bit Windows.

The only real problem was that we tell people in the docs that Visual Studio is optional, but we don't behave that way. We also do different things on 32-bit vs. 64-bit, which is unnecessary. We do tell to define MSVC when you build with Visual Studio, so I changed the native bits Makefile to just set USE_MSVC based on that.

@twall, this is for your, LMK what you think.