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: Ieb8753223dcfb2883797f075b71ca721dd24146d
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 8fc1ac7..8dc3a6a 100644
--- a/meson.build
+++ b/meson.build
@@ -71,6 +71,6 @@
 )
 
 build_tests = get_option('tests')
-if build_tests.enabled()
+if build_tests.allowed()
   subdir('test')
 endif