build: use allowed over enabled or not-disabled

Meson feature options are typically in a tri-state of enabled, disabled,
or auto.  The enabled and disabled functions on an option (from
`get_option`) no longer return true for auto features.  Instead, the
expectation is to use `allowed()` which is true for both enabled and
auto.

Switch all uses of `enabled` to `allowed`.
Switch all uses of `not disabled` to `allowed`.

Change-Id: I013e69a9f45fefdedf3f62c59bd2471ec2c958c1
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index e655f2c..4006075 100644
--- a/meson.build
+++ b/meson.build
@@ -96,6 +96,6 @@
   install: true,
   install_dir: get_option('datadir') / 'dbus-1' / 'system.d')
 
-if not get_option('tests').disabled()
+if get_option('tests').allowed()
   subdir('test')
 endif