autotools features optional python support, which makes some checks
regarding python and pygobject versions and compiles the installed
python files.

This patch adds the equivalent python support in meson.

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

autotools generates the VAPI files for the introspection files and
is missing in meson.

This patch adds support for VAPI generation in meson.

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

The namespace string is used in multiple places in the meson's build
files.

This patch uses a variable which eases the use of this string
through the build files.

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

Since meson 0.43, the `generate_gir` function has a parameter
called `header` which can be used as the name of the header to be
included by the library.

This patch uses this parameter to avoid using the `--include-header`
separately.

The `--identifier-prefix` has also been removed because the
`identifier_prefix` from `generate_gir` is already used.

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

The `libgit2_glib_inc` variable holds two directories, the source
root and also the `libgit2-glib` directory. However, this is not
necessary, because a variable for the source root already exists,
which is called `core_inc`, and the example programs which use
the `libgit2_glib_inc` variable don't need the source root include.

Now the `libgit2_glib_inc` variable only holds the `libgit2-glib`
directory.

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

The convention when naming dependencies is to use the '_dep' suffix,
which helps detecting dependency variables. The variables have been
renamed to follow this convention.

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

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`.

The compiler flags are now only checked and used on debug builds.
To ease the compiler flags checking the `get_supported_arguments`
helper function is 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

The variable holding information about the host system is not
used at all.

This patch removes this unused variable.

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

The gtkdoc option has been changed to follow the meson porting
guidelines[0].

A greater use of different variables is done to avoid any typo when
rewriting the different strings that are needed for `gtkdoc`
function parameters.

When querying the glib dependency prefix, now the variable holding
the previous dependency search is reused.

The list of private headers has also been removed, and now the
list present in meson build from the `libgit2-glib` directory is
reused.

Finally the links present in the generated documentation are fixed
by using the `--html-dir` option.

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

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

Following the meson porting guidelines[0], the `ssh` option has been
changed to be a boolean option.

The options behaviour has also been slightly simplified by moving
the configuration object to the libgit2-glib where the `ggit.h.in`
actually resides.

An auxiliary variable has also been set to avoid writing the file
name twice which can be error prone.

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

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

The build of the introspection data depends on the existence of the
GObject library, the `g-ir-scanner` program, and the build not being
a cross build.

The check to the GObject library has been removed, because it is
required for compiling libgit2. The gir variable has also been
removed because there is no need to store the intermediate state.

Finally, the `build_gir` variable has been renamed to remain
consistent with other similar variables like `enable_ssh`.

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

meson features a module called `pkgconfig` that can be used to
generate pkgconfig `.pc` files.

This patch takes advantage of this module to generate the pkgconfig
`.pc` file.

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

The `includedir` used for the headers of the package itself has been
usually named `pkgincludedir`.

This patch creates a new variable to hold the name along the api
version and uses this to create a new `pkgincludedir` variable
which is going to be used instead of the `real includedir`.

The subdir parameter of install headers has also been changed to
use the path relative to the system header directory instead of
the absolute path.

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

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