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: I8d258c64711ba2cb2cdfb19f9cd827c489a417bd
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/monitor/meson.build b/monitor/meson.build
index 1fa26b8..7ceed5b 100644
--- a/monitor/meson.build
+++ b/monitor/meson.build
@@ -52,7 +52,7 @@
     install: true
 )
 
-if(get_option('tests').enabled())
+if(get_option('tests').allowed())
     subdir('test')
 endif