build: use allowed over enabled

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`.

Change-Id: I934c9ef13747134f41da36ee538d094c72fefdce
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index b7fcebc..ffe9c73 100644
--- a/meson.build
+++ b/meson.build
@@ -85,6 +85,6 @@
            dependencies : deps,
            install : true)
 
-if get_option('tests').enabled()
+if get_option('tests').allowed()
     subdir('test')
 endif