build: Define glib_sanitizers variable to easily get the sanitizers in use

It's an array containing the list of sanitizers in use, normally it
contains a value, but in some cases may have more than one (e.g.
'address' and 'undefined').

And so use it to avoid repeated checks

girepository/introspection/meson.build

@@ -9,7 +9,7 @@ gi_gen_shared_sources = [
 
 gi_gen_env_variables = environment()
 
-if get_option('b_sanitize') != 'none'
+if glib_sanitizers.length() > 0
   gi_gen_env_variables.append(
     'ASAN_OPTIONS', 'verify_asan_link_order=0', separator: ',')
 endif