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: I56a4d75a17ba7afe2104d271a1a4337b8d370475
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 8b65e92..e25c24b 100644
--- a/meson.build
+++ b/meson.build
@@ -77,10 +77,10 @@
 
 subdir('tools')
 
-if not get_option('examples').disabled()
+if get_option('examples').allowed()
   subdir('example')
 endif
-if not get_option('tests').disabled()
+if get_option('tests').allowed()
   subdir('test')
 endif