Fix windows build

Fix windows build

Right now, the Windows build is broken because of NBD.  Using a mingw32 cross
compiler is also badly broken.

This patch fixes the Windows build by stubbing out NBD support until someone
fixes it for Windows.  It also santizing the mingw32 cross compiler support
by replacing the --enable-mingw32 option with a compiler check to determine
if we're on windows or not.

Also remove the weird SDL pseudo-detection for mingw32 using a cross compiler.
The hardcoded sdl-config name is seemly arbitrary.  If you cross compiler SDL
correctly and modify your PATH variable appropriately, it will Just Work when
cross compiling.

The audio driver detection is also broken for cross compiling so you have to
specify the audio drivers explicitly for now.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5046 c046a42c-6fe2-441c-8c8c-71466251a162