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

gio/tests/meson.build

@@ -310,7 +310,7 @@ if host_machine.system() != 'windows'
   }
 
   # LD_PRELOAD modules don't work so well with AddressSanitizer
-  if have_rtld_next and glib_build_shared and get_option('b_sanitize') == 'none'
+  if have_rtld_next and glib_build_shared and glib_sanitizers.length() == 0
     gio_tests += {
       'gsocketclient-slow' : {
         'depends' : [