Here is a patch that updates QEMU to 5.2.0-rc4.

The big change here is that the build system has changed to use Meson
and Ninja "under the hood", although this release still uses
`./configure && make` in a superficial way.

With this patch, the qemu and qemu-minimal packages build and can boot
Guix VMs created with the `guix system` commands.

There are still two open tasks that require assistance:

1) The patch that makes Sphinx build an info manual will need to be
adapted to the meson build system. Ideally, this would be submitted
upstream.

2) The ./configure step is considered "stale" in the build phase and
thus gets run twice. You can add "--trace" to #:make-flags to trace the
Makefile and try to understand what's wrong (shown below). I'm no Make
expert but, AFAICT, there are some PHONY targets that depend on the
output of the configure step, config-host.mak, so it may be an upstream
bug. But I might be misunderstanding it. I asked on the QEMU IRC but
didn't get a useful answer.

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starting phase `build'
GNUmakefile:10: update target 'all' due to: force
echo 'changing dir to build for make ""...'
changing dir to build for make ""...
make -C build -f Makefile 
make[1]: Entering directory '/tmp/guix-build-qemu-minimal-5.2.0-rc4.drv-0/qemu-5.2.0-rc4/build'
Makefile:109: update target 'config-host.mak' due to: /tmp/guix-build-qemu-minimal-5.2.0-rc4.drv-0/qemu-5.2.0-rc4/pc-bios
echo config-host.mak is out-of-date, running configure
config-host.mak is out-of-date, running configure
if test -f meson-private/coredata.dat; then \
  ./config.status --skip-meson; \
else \
  ./config.status && touch build.ninja.stamp; \
fi
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