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: I611ecba901a43acbcca19b93a4dcd9578b570bd8
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 8524d33..059f043 100644
--- a/meson.build
+++ b/meson.build
@@ -44,7 +44,7 @@
             )
             gmock_dep = gtest_proj.dependency('gmock')
         else
-            assert(not get_option('tests').enabled(),
+            assert(not get_option('tests').allowed(),
                    'Googletest is required if tests are enabled')
         endif
     endif