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: Ib0b26af4bd0e8685c83b1b0181f70bfca59ab2bc
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/user_channel/meson.build b/user_channel/meson.build
index b47b27c..701c86a 100644
--- a/user_channel/meson.build
+++ b/user_channel/meson.build
@@ -37,7 +37,7 @@
     description: 'libchannellayer',
 )
 
-if not get_option('libuserlayer').disabled()
+if get_option('libuserlayer').allowed()
     userlayer_pre = declare_dependency(
         include_directories: [root_inc, user_channel_inc],
         dependencies: [