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: Ia2942bcca7a9d47ae5662ab5bc1978c993974482
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 715045c..48efab7 100644
--- a/meson.build
+++ b/meson.build
@@ -72,7 +72,7 @@
   install: true,
 )
 
-if not get_option('7seg').disabled()
+if get_option('7seg').allowed()
   udevdir = dependency('udev', required : false).get_variable('udevdir')
   assert(udevdir != '', 'Cannot find udevdir')
   install_data(['80-7seg.rules'], install_dir : udevdir)
@@ -98,6 +98,6 @@
   'lpcsnoop/snoop_listen.hpp',
   subdir: 'lpcsnoop')
 
-if not build_tests.disabled()
+if build_tests.allowed()
   subdir('test')
 endif