build: Fix requirements for tests and example features

Currently assert checks inside the meson.build files check the
presence of the 'tests'/'example' options by using
'get_option(...).enabled()' expression.
This is a bug since this expression is only true for the 'enabled'
options and in the local build 'tests' and 'example' features are
set to 'auto'.
To correct the issue use 'get_option(...).allowed()' which returns
true both for 'enabled' and 'auto'.

Tested:
On the system without boost "meson setup build" now fails with a
message:
"Boost is required when tests are enabled"

Change-Id: I92b4f5de01f47d27c8fae32b641e4b2083fe5b25
Signed-off-by: Konstantin Aladyshev <aladyshev22@gmail.com>
diff --git a/example/meson.build b/example/meson.build
index c7ebf61..4911a67 100644
--- a/example/meson.build
+++ b/example/meson.build
@@ -14,7 +14,7 @@
 endif
 
 assert(
-    not get_option('examples').enabled() or has_asio,
+    not get_option('examples').allowed() or has_asio,
     'Boost is required when examples are enabled'
 )
 
diff --git a/test/meson.build b/test/meson.build
index 2efdfeb..1c9f7aa 100644
--- a/test/meson.build
+++ b/test/meson.build
@@ -55,7 +55,7 @@
 endforeach
 
 assert(
-    not get_option('tests').enabled() or boost_dep.found(),
+    not get_option('tests').allowed() or boost_dep.found(),
     'Boost is required when tests are enabled'
 )