fix for broken build due to nonblocking IO

Here are two new patches, which attempt to fix the QEMU build for the dependency on pthreads introduced by nonblocking IO. They are on the nonblocking IO page:

http://people.brandeis.edu/~jcoiner/qemu_idedma/qemu_dma_patch.html#install

The first is a quick fix to the QEMU's 'configure' script and 'Makefile.target'. On most unixes this will fix the build.

On MinGW, the patch makes the configure script assume that the pthreads headers and DLL live in certain locations. The second patch (for MinGW only) applies to the pthreads-w32 distribution, so that it puts those headers and that DLL into those certain locations when you run 'make install'.

The MinGW stuff here is pretty lame. It may not be better than nothing, but I don't really know what the right way is. That's why the build patches are separate from the C code patches -- they may be going down the wrong path, especially on MinGW.

-- John