Create a config.vapi for config.h

Currently, a lot of our build variables that are defined at
configuration time, are spread out across the code base, often declared
as `extern` which can break when moving around sections of the code
across files.

This commit introduces a "Config" namespace which basically maps to the
definitions in `config.h`, but allows us to properly access them too
from the Vala source code.

By doing so, it helps us to more explicitly see where we rely on a
build variable from this file (which should be obvious from the `Config'
namespace).

To make it ourselves a bit easier in Meson too, we can declare an
internal dependency, which helps ensure that we pull in the dependency
where needed.

test/engine/imap-engine/imap-engine-generic-account-test.vala

@@ -55,7 +55,7 @@ public class Geary.ImapEngine.GenericAccountTest : TestCase {
         this.local_account = new ImapDB.Account(
             config,
             this.tmp_dir,
-            GLib.File.new_for_path(_SOURCE_ROOT_DIR).get_child("sql")
+            GLib.File.new_for_path(Config.SOURCE_ROOT_DIR).get_child("sql")
         );
         this.local_account.open_async.begin(null, this.async_completion);
         this.local_account.open_async.end(async_result());