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

meson related:
* convert unit tests
* bugfixes for mtest2make
* miscellaneous bugfixes
* dead code removal and configure cleanups
* oss-fuzz fixes
* msys fixes

# gpg: Signature made Tue 08 Sep 2020 10:43:27 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: (45 commits)
  docs: update build system documentation
  meson: remove linkage of sdl to baum
  meson: Convert undefsym.sh to undefsym.py
  fuzz: Add support for custom fuzzing library
  meson: specify fuzz linker script as a project arg
  oss-fuzz: fix rpath
  configure: update dtc submodule
  docs: suggest Meson replacements for various configure functions
  configure: drop dead variables and functions
  configure: do not include dependency flags in QEMU_CFLAGS and LIBS
  meson: get opengl compilation flags from OPENGL_CFLAGS
  meson: get glib compilation flags from GLIB_CFLAGS
  configure: do not look for install(1)
  configure: remove unnecessary libm test
  configure: move -ldl test to meson
  meson: keep all compiler flags detection together
  configure: move disassembler configuration to meson
  Makefile: inline the relevant parts of rules.mak
  Makefile: remove dead variables and includes
  meson: compute config_all_devices directly
  ...

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