Merge remote-tracking branch 'remotes/bonzini-gitlab/tags/for-upstream' into staging

New build system, with "fake in-tree builds" support.

Missing:
* converting configure tests
* converting unit tests
* converting some remaining parts of the installation

# gpg: Signature made Fri 21 Aug 2020 11:33:35 BST
# gpg:                using RSA key F13338574B662389866C7682BFFBD25F78C7AE83
# gpg:                issuer "pbonzini@redhat.com"
# gpg: Good signature from "Paolo Bonzini <bonzini@gnu.org>" [full]
# gpg:                 aka "Paolo Bonzini <pbonzini@redhat.com>" [full]
# Primary key fingerprint: 46F5 9FBD 57D6 12E7 BFD4  E2F7 7E15 100C CD36 69B1
#      Subkey fingerprint: F133 3857 4B66 2389 866C  7682 BFFB D25F 78C7 AE83

* remotes/bonzini-gitlab/tags/for-upstream: (152 commits)
  docs: convert build system documentation to rST
  meson: update build-system documentation
  meson: avoid unstable module warning with Meson 0.56.0 or newer
  meson: convert po/
  meson: convert VNC and dependent libraries to meson
  meson: move SDL and SDL-image detection to meson
  meson: convert sample plugins
  meson: replace create-config with meson configure_file
  rules.mak: drop unneeded macros
  meson: convert check-block
  meson: build texi doc
  docs: automatically track manual dependencies
  meson: sphinx-build
  remove Makefile.target
  rules.mak: remove version.o
  meson: convert systemtap files
  configure: place compatibility symlinks in target directories
  meson: link emulators without Makefile.target
  meson: plugins
  meson: cpu-emu
  ...

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>