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: I29e5c0eba3337ed9a319f00ccb976e28233fb515
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 5506203..0a0db8c 100644
--- a/meson.build
+++ b/meson.build
@@ -210,7 +210,7 @@
     install_dir: get_option('datadir') / 'dbus-1' / 'system.d',
 )
 
-if not get_option('unit-failure-monitor').disabled()
+if get_option('unit-failure-monitor').allowed()
     executable(
         'phosphor-unit-failure-monitor',
         ['fail-monitor/main.cpp', 'fail-monitor/monitor.cpp'],