Add support for building with meson, enabled by '--with meson' so that
we can regularly test the whole build+test+install procedure with
meson. I compared the RPM contents of NM, NM-libnm, NM-libnm-devel
packages and they match the autotools ones. It's also faster:

  $ time contrib/fedora/rpm/build_clean.sh -g -Q -f

  real    3m54.239s
  user    11m15.000s
  sys     1m28.456s

  $ time contrib/fedora/rpm/build_clean.sh -g -Q -f -w meson

  real    3m9.938s
  user    9m5.225s
  sys     1m4.392s