Normally it shouldn't matter too much, but the GIR parser apparently
doesn't like it:
```
/home/niels/gnome/libsecret/libsecret/secret-schema.h:75: syntax error, unexpected ';' in ';' at ';'
/home/niels/gnome/libsecret/libsecret/secret-prompt.h:78: syntax error, unexpected ';' in ';' at ';'
/home/niels/gnome/libsecret/libsecret/secret-value.h:54: syntax error, unexpected ';' in ';' at ';'
/home/niels/gnome/libsecret/libsecret/secret-service.h:307: syntax error, unexpected ';' in ';' at ';'
/home/niels/gnome/libsecret/libsecret/secret-collection.h:176: syntax error, unexpected ';' in ';' at ';'
/home/niels/gnome/libsecret/libsecret/secret-item.h:194: syntax error, unexpected ';' in ';' at ';'
```
g_autoptr() is a macro that was defined in GLib 2.44 that allows for
basic auto-cleanup of variables. One way to add this kind of support
would be through the use of e.g. `G_DECLARE_DERIVABLE_TYPE()` for our
declarations, but this would consitute an ABI break (due to the
`...Private *` field in the public structs). Instead, we can use
`G_DEFINE_AUTOPTR_CLEANUP_FUNC` to manually declare this.
This commit also bumps the minimally required GLib version to 2.44
* C callers need to use libsecret-0 pkg-config file for stable and
libsecret-unstable for unstable stuff.
* Vala callers need to '--pkg libsecret-unstable' for unstable
* GObject Introspection callers need to use the SecretUnstable package
* Death by a thousand paper cuts from gir and vapi not liking
the fact that the secret.h file was not usable uninstalled
and installed in the same way.
