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: I86623dd968ce368a5349faf3e97728c02549c2bd
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 0dc6209..b9dbd1b 100644
--- a/meson.build
+++ b/meson.build
@@ -75,6 +75,6 @@
   subdir('example')
 endif
 
-if not get_option('tests').disabled()
+if get_option('tests').allowed()
   subdir('test')
 endif