doc: Update mingw cross compile instructions

doc: Update mingw cross compile instructions

The "Cross compilation for Windows with Linux" section of qemu-doc.texi
still instructs the user to use 'configure --enable-mingw32'
even after the option was removed in Aug 2008:
http://git.savannah.gnu.org/cgit/qemu.git/commit/?id=cd01b4a312248dd4e12c3d389d1a349cea4015d8

This documentation only change updates the instructions to:
* Remove use of '--enable-mingw32' in the configure example
* Correct the 'sdl-config' script name
* Remove references to i386-mingw32msvc.tar.gz which no longer exists in
  recent SDL releases
* Document the zlib dependency

Signed-off-by: Scott Tsai <scottt.tw@gmail.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>