meson provides two options related to debug options. The first one
is the build type, which among others, can be `debug` build or
`debugoptimized`. The second option which is called `b_ndebug` can
be used to enable or disable assertions.

The meson guidelines[0] recommends the use of options present in
meson, so the combination of these two options has been used to
replace the `debug` option.

- `debug` build sets the `LIBGIT2_GLIB_ENABLE_DEBUG` macro, which
  is equivalent to `debug=yes`.
- `debugoptimized` build sets the same macros as `debug`, but also
  set `G_DISABLE_CAST_CHECKS`, which is equivalent to
  `debug=minimum`.
- If build is not a `debug*` build it sets `G_DISABLE_CAST_CHECKS`
  and `-G_DISABLE_CHECKS`.
- If `b_ndebug` is true it sets `G_DISABLE_ASSERT`.

To ease the compiler flags checking the `get_supported_arguments`
helper function is also used, which implies bumping meson version to
0.43.

https://bugzilla.gnome.org/show_bug.cgi?id=792699

[0] https://wiki.gnome.org/Initiatives/GnomeGoals/MesonPorting